If your son does not amend his return, and you file by mail claiming him as a dependent, your return should be processed as normal. However, at some point in the future (and it may be a year from now), the IRS will send you both correspondence which will enable them to determine which one of you actually should get the exemption for your son. Whoever does not will lose the exemption and may owe additional taxes.
If you believe you should get the exemption (it appears that you should from what you stated), he should amend his return now. You will still have to file your return by mail to claim the exemption from him, but this will avoid the correspondence from the IRS.
