What is he having the specialist diet for exactly?
I agree with the others, deffo a vet check first off, could be a dental prob as Christine mentioned, specially as you say he's spitting out the biscuits, mind you, many prescription diets are notorious for being unpalatable LOL! so its more important that he eats something, even if its not the specialist diet.
The Feliway plug ins might help with the settling in process too.