It takes some time for RES to get used to their environment and until they aren't stressed, they will not eat. It is very important to make sure he is in the correct habitat with proper water/basking temps, place to bask under heat light and a UVB light, plenty of swimming room, good filter, places under water to hide/rest on:fake /real plants. If everything is correct in the set-up give him a few days to warm up to things. If the set-up isn't correct, be sure to do your best to set up his habitat correctly as soon as possible.
Are there any signs/symptoms of illness? If not then just give him some time to get used to things. Enticing him to eat with something smelly (bits of tuna packed in water) might work for a while until you can get him to eat the pellets/veggies he should have.
