Was it just a marble or one of those Magnetic marbles? And only the one?

He will have a sore belly as it passess through i would think, so maybe give him some panadol to help ease that.

Take him to the Dr/Er if he starts to get anymore symptoms: Pain increases/temps/groaning/moaning/guarding... me personally would give 13health or ED a call and explain to them and ask for their advice on what to do right now, they might want you to bring him in.
It was glass, and not magnetic. Phew, at least I don't have to worry about that, I've heard such horror stories about children swallowing magnets and rupturing their intestines.