you may want to find out what the petstore did to get him to eat. Did they give him a live pink? f/t pink, cut pink, brained pink?
I'd find out what they did to get him to eat for them then follow the same regiment for a few feedings before you try feeding him "just" a f/t. Good luck with your little guy
One other thing. I'd treat him like a new buy, and not try feeding him for atleast 5 days after you took him back. That way you can be sure he has acclimated to his surroundings again and you can treat him like a new snake.