Give him some time to get adjusted to such a big tank. I brought a dumbo halfmoon betta from PetsMart and it did something very similar. This one likes to go to the top to breathe but he is active and swims around and begs for food. One thing you have to realize is that poor fish is conditioned to life in a small place and you just dumped him in a five gallon tank. I would try turning the current down to the lowest setting (some bettas are not good at swimming in a current - mine literally rolled around and scared the crap out of me) and just keep on trying to feed him and cleaning out the ones he doesn't eat. It took my fish a very long time to notice that the food pellets were floating on the top of the tank and he eventually started eating them. If they had a small bowl (and it sounds like they probably didn't clean it as often as they should) he might be suffering from some ammonia poisoning. Does his body look okay (finrot, dots, etc)? He might need sometime to acclimate and recover from the bowl.

Also, do you have a heater in this tank? Maybe he's a bit cold. I just am asking. Was he very active in the bowl?