If he can eat live feeders, then yes I would feed him crickets. I get the large pre-adult for our bigger dragons. Papa Smurf can't chase them down, so I had feed them, one at a time. If your dragon can go after them and eat them, that's even better. I really think that feeding whole food - live feeders - is the best for them if they are able to eat them. As with any recovering animal, do gradually introduce the new types of food. For Papa, I gave him about 3 crickets the first time, then increased the number by a few each feeding. It's just that I've heard from other, more experienced people, to be slow and gradual with all changes as lizards are slow to change and recover.
Just wait until you see how he is with that UVB - I am excited for you.
I know other people have taken in sick and malnourished/neglected animals and they don't always survive. I had someone one here tell me that they had dragons like my Papa Smurf live about 1-1/2 years after being rescued. We just don't know how their organs are and at what age the neglect started. Maybe they had UVB while they were young and growing, or maybe they didn't. I would think that would affect how their organs mature and function. :?: So there are no guarantees. Shoot, there aren't any guarantees with people either. Any healthy appearing human can have cancer ticking away inside them, or something else. We just have to hope and do the best we can!