Is my bearded dragon sick?

I just adopted a 4 year old bearded dragon. He is a very picky eater and does not drink water on his own. He is under weight for sure but still pretty active. He is very pale in color. He has a UVA, UVB and and I just bought a heat lamp tonight. I do not have a thermometer but I know the last few mornings he was cold so I am hoping the heat lamp will eliminate that. He has a 55 gallon tank and I ensure that there is an area for him to go if he gets to hot. The previous owner was not using the heat lamp at night for quite sometime but I was told by the pet store to start it again and it might help him want to eat again. I was also told to spray him each day with water to ensure he is not getting dehydrated. His eyes are yellow but I have been told it could be doing. I am offering crickets every few days and veggies like kale, squash and zucchini daily even though I was told that he refuses to eat them. He was previously fed sweet potato which I will offer as well but I am trying to increase his diet options. I was told by the previous owner that they did superworms but they had to be fed by hand and it takes alot of time to get him to eat. My son will be devastated if he dies. Is there anything else I can try? How do I know if it is too late?