It definitely sounds like he's never eaten before you got him and with the stress of a pet store, it might take a bit. I got 2 babies fresh out of the egg from a show that both were problem feeders. I think that stress on a newly hatched baby can cause a delayed feeding response but with time, you can get them to start eating. I would offer each week and then every other week, assist feed the day after a refusal. You don't want to do an assist feeding right after they refuse a meal because they may correlate the stress of the assist with the offered food. Keep a close eye on his weight but make sure you keep offering him the chance to take his own meal. Eventually, he will surprise you though it may take time.