Why do children do this? . I suspect mine will do the same. He's just discovered his nose.
Unfortunately I've no pearls of wisdom to offer except the ones already offered. It could just simply be a case of him discovering that there's a hole there and he can get a finger in it.
More likely to be thread worms though - they are easily picked up and very common. You can treat it with some stuff from Boots (the name of which escapes me).