Antiparasitics are really very harsh on their stomachs, and it does sometimes take a very long time for their systems to get back to normal. The probiotics you're giving to him is step one, I'd be giving him the acidophilus every day...What form of acidophilus are you giving him? Is it a liquid acidophilus made as a supplement, or is it just soy yogurt? You may want to try a general probiotic mix that contains other strains in addition to the acidophilus, like lactobacillus and others. Sometimes the acidophilus isn't enough to get their stomachs feeling better and to restore the "healthy bacteria", and you need a mix...
I have had HUGE LUCK with my rescue beardie that had been severely impacted and had not only not had a bowel movement for months, but also hadn't eaten in months, with a specific probiotic tablet that also contains digestive enzymes. I actually started using it years ago when I used to breed and hand-raise budgies and cockatiels. Whenever they were treated with antiparasitics or antibiotics they would stop eating and lose a lot of weight. After trying numerous different things, I tried a combination of this probiotic/digestive enzyme supplement and Bee Pollen.
I buy both the probiotic/digestive enzyme and the Bee Pollen at Walmart. The brand is Spring Valley, it's the brand that Walmart carries the most of in their vitamin/supplement section, and they use a yellow and green labels with green bottles. The Bee Pollen is simply called "Bee Pollen", and comes in a bottle of 100 capsules that are 550mg each. The capsules can just be pulled apart, and I add a pinch to the babyfood or greens slurries I force feed.
The Spring Valley brand probiotic/digestive enzyme supplement is called "Probiotic Multi-Enzyme" and underneath that title it says "Digestive Formula with Active Acidophilus Cultures". It comes in a bottle of 200 tablets. They are small, white, round tablets that I crush to a very fine powder, and I typically add 2 pinches of this to the slurry as well.
Using digestive enzymes has been commonplace in avian medicine for quite some time to not only promote appetite but also to cure crop stasis, slow crop, yeast infections, and also to help break-down impactions and clear out the digestive tract. I have successfully used these exact tablets in week old baby birds whose parents fed them crop milk in their first week of life that was too thick and caused all kinds of issues. The probiotics along with the digestive enzymes (to sum it up) basically break up any bad stuff that's lingering, clear everything out, replace the good stuff, reduce inflammation, and get them feeling hungry again. The Bee Pollen is another appetite stimulant commonly used in reptile and avian medicine, so you're attacking from all angles with natural, plant-based supplements that make them feel better. These digestive enzyme tabs contain Amylase, Lipase, Cellulase, Protease (papaya), Pancreatin, Pepsin (helps immensely with inflammation and acid in their stomachs), and I believe Bromelain as the digestive enzymes. Then the lactobacillus acidophilus is also included in the same tablet for easy dosing.
I typically use 2 pinches of the Probiotic Multi-Enzyme, 1 pinch of the Bee Pollen, a pinch of Calcium with D3, and a pinch of Reptile Multivitamin without Calcium, and mix these supplements with Babyfood, the type depends on whether or not they are impacted as well. I always use an all-natural babyfood brand, like Beech-Nut ($0.75 a jar), that basically only use the fruits, vegetables, and water. There are no added sugars or sodium. If the Beardie is impacted I'll use Beech-Nut Prune babyfood and I'll also add a tablespoon of raw, canned pumpkin in with it, and then I'll mix in the supplements thoroughly with a couple tablespoons of unflavored Pedialyte. If the Beardie isn't impacted, I may still use the Beech-Nut Prune babyfood because they really seem to like it and I'd rather keep them cleared out, but I won't add the raw, canned pumpkin.