Hi, I noticed your cute little guy has full colors, so he's mature. I'm sorry you got bit, it really hurts! First, he's probably just a bit freaked out by being in a completely new environment from the store he was used to. Adult Poi's first reaction is to bite. As you now notice, they have a big strong beak for their body size! Mature male RB are known for being a bit nippy. They can bite pretty deep if they want to. Just let him settle in and as hard as it is, go hands off a little more for now. It's probably fear biting (although he's tame). Like thekarens said, just ignore the bite and put him away for the time being. As time goes on, and he warms up, pet him gently and reassure him, and little by little you should see him become the little buddy you met at the store. Red Bellied are outgoing birds, so it shouldn't take long.
Later on, when he's completely comfortable, you can (stay calm) but stare him directly in the eye and say a firm NO. Also showing him you're confident, laddering step ups and so forth... But later after he's settled.
Sorry I'm pressed for time right now, hope that made any sense, I'll check back later! Good luck. I think you'll do just fine.
