Sorry for all the questions, just trying to cover the important things, but I guess I have more questions.
It sounds like you're using a Repti-Sun fluorescent bulb then, do you know if it is the 10.0, 8.0 or 5.0? What is the distance from the UVB to his basking area?
Good to hear that he is active during the day and that the basking bulb is a white one. What size tank is he in?
Yes, you can feed crickets and since he is about 18" long, he should be able to handle the large ones with no problem. Since he is so thin, I would start feeding him some crickets and you could continue feeding a few wax worms but sometimes once they start eating those, they don't show must interest in other feeders again. I'm still not sure if he is under a yr or older, but you could give him as many crickets as 50 or even more a wk, 50 is the amt recommended for adults but since you're trying to help him gain more wt and you think he has been lacking food from his prior care, you should start with about 40 a wk and go up from there but I wouldn't recommend over 70 a wk for now unless you're going to supplement with the chicken & squash baby food. Once he starts putting on wt, then cut back to the 50 crickets a wk.
The calcium should have D3 but be phosphorus-free and you should be dusting either his live food or greens 3 to 4 days a wk and with vitamins 2 days a wk.
Does your digital thermometer have a long wire & probe end? If it doesn't, then you will need to get one of those or a temperature gun. The stick-on thermometers are inaccurate & can be off as much as 20 degrees.
Giving him a
bath at least 3 to 4 times a wk would be a good idea as proper
hydration helps with their activity level, pooping and appetites. A 15 to 20 min
bath is a good amt of time to be beneficial.
*Edit: I just noticed that the tank is 75 gallons, that's a good sized tank. The 150 watt bulb should be ok but if you're not using a digital with probe end, I suggest you get one to check the temps. The vita-sand can be dangerous as it contains vitamins & encourages them to lick it which can cause an impaction. You might want to think about changing to children's play sand or a solid substrate instead.