Ideally a fiddle. But that takes a lot of learning.

Mouth organs are tough, small cheap(ish), and easy to learn by yourself, and to play with others. And he can sing the verses and play in between, or when someonme else is doing the singing or playing.

Bones rather than bodhran - smaller, tougher, less obtrusive, and ppeople always ask you to show them how to play them. And it's the only instrument you can play while you're pouring a drink down your throat.

Penny whistle because it's cheap, small, tough and potentially the most beautiful of all.

So I'd suggest a couple of mouth organs (maybe a double sided one with two keys - or a chromatic one), plus a pair of bones and a D whistle.