I'd go with MacOne's suggestion with one addition. I'd buy him a membership of the best, local, air rifle club you can find, having visited a number of them. Then he can go along for a couple of weeks, see what people are shooting, have a coffee and a chat with some of the members, perhaps try some club rifles and figure out what type of shooting he wants to do and what he'd like to shoot.
Incidentally, I'd splash out and buy one for yourself and go with him.