I am sorry to say the people at Petco gave you some dangerous and very bad information,You have some serious husbandry issues which can be causing the Dragon the problems you are seeing. The sand can cause some major issues with a Dragon. The Desert Blend is walnut shell and can perforate the intestinal tract and can also cause serious issues this is about the Desert Blend: Graphic: http://mrskingsbioweb.com/beardeddragngrossanatomy.htm I I suggest using a solid substrate like tile, cage carpet, slate or paper.
Loose substrates can also cause skin problems and corneal ulcers in the eyes.
The basking area should have 110*-115*F, 90's on the warm side and the cool side of the tank in the 85* range. This is important for digestion and passing of stool. If the temps are not correct, this can also cause a problem. You need to have 2 lights, a day time basking bulb as well as a UVB. I recommend a reptisun 10.0 florescent tube. The UVB bulb is needed to stop Metabolic Bone Disease. The UVB needs to be changed every 6 months. Coil UVB bulbs can injure the eyes.
Compact UVB problems: http://www.uvguide.co.uk/phototherapyphosphor-info.htm
The UVB bulb should not have any plastic between it and the Dragon, it needs to run the length of the tank and be no more than 12" from the Dragon. Both lights should run daily for 12-14 hours a day. UVB cannot penetrate glass so sitting by the window, does not supply UVB rays which are needed to metabolize calcium.
The heat rock should be removed asap. The diet is not the best as lettuce does not hold the proper nutritional value. It is mostly water. You should be feeding Collard greens, mustard greens, dandelion greens, escarole and curly endives. Butternut squash and various berries are great for a treat. The Veggies should be put in fresh daily, and done about an hour after lights on. They need 20% veggies daily and 80% live prey under a year of age. This is a chart for foods and is color coded: http://www.beautifuldragons.com/Nutrition.html
The live prey should include crickets, silk worms, roaches, goliath worms and
phoenix worms. Meal worms ouuter shell are too high in chitin for a Dragon to digest.Super worms and wax worms are like giving the Dragon candy. All veggies and prey should be dusted with calcium daily 5 days a week and vitamins 2 days a week.
It is very important especially with a young Dragon to be dusting the prey with a calcium supplement daily. The Dragon is showing signs of Metabolic Bone Disease with the tremors: http://exoticpets.about.com/cs/reptilesgeneral/a/metabolicbd.htm.
I would get him onto paper towels immediately and get rid of the walnut shell asap. You have to start getting calcium into the Dragon. I will give you some first aid methods.
Please give the Dragon a soak in a 50/50 warm water soak with Pedialyte and some calcium powder. This will help. Get the new UVB and get calcium started asap. I can locate a Herp Vet as I suspect you may need one if we cannot get the tremors stopped. The Dragon should also have a fecal done to rule out parasite issues because we are seeing a lot coming from the Pet Shops with issues.This is a link for AZ Herp Vets: http://www.anapsid.org/vets/arizona.html