It is possible to file amended tax returns.
Currently working for a client who has filed his 2005 US tax return under the assumption his full income was taxable in the US. It now turns out (after he hired us) that part of his income was subject to Dutch tax, so he now wants to claim back US taxes. We are in the process of preparing the amended tax return so he can claim back taxes.
Looking at the above, I see no reason why this should not be possible for your son. Please note, however, I am a Dutch tax expert, not an US tax expert, so I could be wrong on this one.