N.K.
Are you/they sure he swallowed only one magnet? If it is more than one, or if he swallowed another metal object, it can be very dangerous!
Belly aches and flu-like symptoms do not sound good at all!
Read this:
http://www.sciencedaily.com/releases/2008/02/080205200644...
I would call the Children's Hospital right away and ask for advice, and even take him there as soon as possible!