Yoshi, is showing signs of MBD which is Metabolic Bone Disease. This can ocur from lack of UVB and a calcium supplement. This is some information on MBD: http://exoticpets.about.com/cs/reptilesgeneral/a/metabolicbd.htm THe UVB light you have is way to weak. These Dragons need a Reptisun 10.0 Linear tube. The temps under the basking light should be 105-110, the warm side should be 90's and the cool side mid 80's.
The UVB need to work with a plain calcium supplement. The UVB metabolizes the calcium into D3 in the body. The weakness in the legs and flipping over are all signs of MBD. I suspect he may also have parasites. Most of the Dragons coming out of the Pet shops do.
Diet should be 80% prey which includes dubia roaches, crickets, silk worms and Phoenix worms aka calcium worms. No Meal worms or super worms for dragons. The Veggies should be 20% including collard greens, mustard greens, dandelion greens, and butternut squash. over a year old the percentages change to 80% veggies and 20% live prey. This is a diet sheet color coded: http://www.beautifuldragons.com/Nutrition.html
Your Yoshi will eventually need a 40 gallon breeder tank.
He will to have a Herp Vet check and if you supply a state I can locate some for you. He will need to be put on a high potency calcium and most likely need to be treated for parasites if some are found in the stool. I would like to start some first aid for him. First we need to change the type of water you are giving him. Distilled takes out the electrolytes, so get him on purified or spring water. We need to give him a soak in a shallow container of water and support him in the soak. To this soak put a pinch of calcium in the soak. Reptiles absorb fluid through the Vent area. Next if he is not eating we need to start supplemental feeding. Please let me know and I will give you a formula on that. WE need to get him under a stronger UVB light as soon as you can. He will have to have food dusted with calcium 5 days a week and 2 days with a Vitamin.
Please read though this information and hit reply with questions an a State to locate Herp Vets.