I assume you don`t have broadband or that it`s not the same ISP. If it`s a wireless router you can configure it without connecting to your phone line. You would have to do this manually so you need to post back your son`s ISP at home, the make and model of the router. Someone will post back the instructions - I`m out for the afternoon but will check back.
This will only set up a connection by ethernet cable or wireless to the router but he should then be able to plug it in at home and it will work immediately.