I wouldn't give any medications until you know what's wrong. Reptiles are very slow to absorb meds, which means they can linger for awhile, causing unwanted damage if not dosed correctly. As for the dog/cat food, I imagine it's hills science diet A/D food? It's used commonly for reptiles as it's an emergency soft food sort of thing. Syringe feeding is NOT the same as force feeding. Syringe feeding is putting something in a needle-less syringe (baby food works well, you can do a mixture of squash and chicken baby food) and then drip it on the tip of his nose. Most dragons like the squash and will lick it off easily. If he's off food and losing weight consistently, I would highly recommend syringe feeding him to get in some sort of nutrients. The baby food is a great option and you can try just squash or mix it with chicken or turkey to get some protein into him.
If he's losing weight regularly and not eating, I would immediately think parasites. They make the dragon feel ill enough to not want to eat and then the parasites slowly eat away at the dragon. The fecal should show any parasites. If he does have parasites, make sure to come back here and list which ones he has so people can give you recommendations on meds and dosages. I never trust my vet for that sort of thing. They often prescribe harsh medications. Tracey (Drache) is great at dosing reptiles for their meds.
Lastly, I would encourage you to get him tested for ADV (Adenovirus). You can get that done through here:
http://www.thepats.info/sign_up.html
Tracey runs this project. Knowing the ADV status of your dragon will help you understand his needs in the future. ADV could also be a cause of his symptoms or a reason he is more prone to parasites if he has them.
My dragon has ADV and I give him a daily slurry to keep him healthy and regular. It's made the WORLD of difference knwing his status and treating him as an ADV positive dragon. His health has improved so much with natural supplements like the bee pollen. This is what I give my dragon every day:
1. 3-4 Tiny Scoops Pangea (any flavor. My dragon LOVES the taste of this stuff and will eat anything mixed in it). I'll be getting this one after I finish my current bag because it's made with organic insects and fruit instead of all fruit, so it suits a dragon a little more:
https://smile.amazon.com/Pangea-Fruit-Insects-Crested-Complete/dp/B019P0KBJA/ref=sr_1_4?ie=UTF8&qid=1484164015&sr=8-4&keywords=pangea+crested+gecko+food
2. 3-4 Tiny Scoops Repashy Veggie Burger - This is to give a little extra protein and veggies instead of having an all fruit-based slurry:
https://smile.amazon.com/dp/B00FJE0MQO/ref=twister_B017TFCX62?_encoding=UTF8&psc=1
3. One Tiny Scoop of Milk Thistle Seed Powder - Helps detoxify the liver, boosts-immune function, and has anti-inflammatory and anti-fungal properties:
http://www.beardeddragon.co/index.php?route=product/product&product_id=623&search=milk+thistle
4. Sugar Free Electrolyte powder (I follow the solution instructions on the packaging, but it make it in 1/2 or 1/4 the dose. I make about 30mL and keep it in a little bottle in the fridge and add 1-2ml of the liquid to his slurry) - Helps with keeping them
hydrated and also contains water soluble B vitamins to boost energy metabolism:
http://www.beardeddragon.co/index.php?route=product/product&product_id=255&search=electrolyte
5. Serrapeptase - To get the dose, take your dragons weight and divide by 1000. For example, my dragons weighs 327g, so 327/1000 =.327, and that's how many mL he gets, .327mL. Serrapeptase is one of those magic things that helps everything, but it thins mucous for respiratory infections and helps prevent bacterial infections in the lungs. It also reduces inflammation so things can heal properly and are less painful, and it digests non-living tissue and breaks down fibrin in scar tissue. :
http://www.beardeddragon.co/index.php?route=product/product&path=61&product_id=399
6. 2 drops Vitamin B - boosts energy metabolism and aids in muscle and nerve function and growth. :
http://www.beardeddragon.co/index.php?route=product/product&path=61&product_id=120
7. 1 Tiny Scoop Bee Pollen - Another wonder supplement, only substance in the world that contains every element in the human body. Essentially an all-natural multi-vitamin and super drug. Also helps immune system function. Bee Pollen contains at least 22 amino acids, 18 vitamins, 25 minerals, 59 trace elements, 11 enzymes or co-enzymes, 14 fatty acids, 11 carbohydrates and approximately 25 % protein. Bee pollen is extremely rich in carotenes, which are metabolic precursors of vitamin A. It is also high In B complex and vitamins C, D, E and Lecithin.
http://www.beardeddragon.co/index.php?route=product/product&product_id=272&search=bee
8. Collodial Silver - I follow the directions on the package, but the place I buy from no longer sells it, so if are interested in it, I would PM Drache as she works with the stuff in helping find a cure for ADV and is great with helping figure out a dosage. Collodial Silver is one of those cure-all remedies, it's ionically very small, so it can absorb directly into the blood stream and organ tissue instead of having to pass through the gut. A lot of people will use it in a nebulizer when dragons have URI's. I use it as it has given some dragons a negative ADV test result when given on a schedule, and my dragon is ADV positive. He gets 15 days in a row, then gets the rest of the month off.
9. Probiotics - I give .2ml per 100g body weigh, but healthy dragons can have just .1ml for daily use while sick ones up to .4ml for a shorter period. Probioitcs are great when dragons have things like ADV destroying their healthy cells or are on medication. It's also just a good daily thing to take, as most people could use more good gut bacteria:
http://www.beautifuldragons.com/Acid.html
10. 2-3 Tiny Scoops Alfalfa powder - Added in just so he always gets some good greens in him as he doesn't really like salad:
http://www.beardeddragon.co/index.php?route=product/product&product_id=135&search=alfalfa
11. Liquid Calcium. He doesn't like when I dust his insects, so I give him .1ml per 100g as a liquid.
12. Psyllium Powder - He has a history of diarrhea so the Psyllium helps keep everything bound together and moving good:
http://www.beardeddragon.co/index.php?route=product/product&path=61&product_id=163
13. Black Cherry Concentrate - Black Cherry is used to help with gout by preventing gout crystals from forming in the joins or tissues of the body. He doesn't have gout, but I give occasionally as a preventative and to add flavor. I add about 3-4 Tiny Scoops of this to the 30mL of the Electrolyte mixture I make up so he gets 1-2mL of this liquid mixture.
Supplies I use:
Tiny Scoop (this is what I mean for the Bee Pollen, Alfalfa, etc.):
http://www.beardeddragon.co/index.php?route=product/product&product_id=327&search=tiny+scoop
3ml Feeding Syringe:http://www.beardeddragon.co/index.php?route=product/product&product_id=294&search=syringe
1ml Syring: I use this for dosing out all the liquid supplements.
http://www.beardeddragon.co/index.php?route=product/product&path=60&product_id=114
It sounds like a lot, but it makes about 3-4mL of a slurry that he loves eating. This slurry changed my dragon. He eats more greens now, on his own. He went from pooping diarrhea once a week to having perfect little regular poops once a day! He actually runs around his tank now and is super active. He's a completely different dragon, for the best. And he's been gaining weight again! Knowing your dragons ADV status can really change the care outcome.