You might want to think about getting a smaller watt bulb for basking. A 100 watts on a 10 gallon tank seems to me to be too much. I use a 40 watt on my 50 gallon during the summer when its warm. Either that or upgrade your tank to a much larger one. He really needs to be able to thermoregulate his temp. The cool end should be 75 - 85. He could be really dehydrated if your temps are as high as they sound. You might try some cool water dripped on his nose and see if he'll lick it off. Also, you don't need to heat the tank at night unless its colder than 65.
I am also a strong proponent of squash baby food. Feed it to him with a small syringe (like an isulin syringe) or with an eye dropper. Squash is a staple and he can eat as much as he wants. It also has a lot of liquid in it and will help him stay
hydrated. Both of my beardies love it and it can help you through those times when you aren't sure what's happening.
The bulbs that provide basking and UVB are called Mercury Vapor Bulbs and you can't use them on a tank that small - 40 gallon breeder is the smallest they are recommended for. I would recommend the Repti-Sun 10.0 flouresant tube bulb for the UVB (get them from Pet Mountain - $20 - much cheaper even with shipping than at a pet store) and an incandescent light bulb or small flood for the basking light. Don't purchase the Repti-glo or a compact flouresant light - they can both cause issues with the beardies eyes and can make them lethargic and sick. Also, if you don't have one yet, buy an Accurite digital probe thermometer from Walmart ($12) - it allows you to take temps on the cool side and on the basking spot at the same time.
I know this sounds like a lot, . . . it probably also conflicts with what the petstore told you, . . . but you don't want to go through the heartache of losing your beardie. We lost Ash, our first beardie, partly due to misinformation and lack of knowledge on our part, and it wrecked my nine year old son. It was like he lost part of his soul that day. I don't want to see anyone have to go through that again.