You can claim him as a dependent IF: 1. you paid over one half his living expenses (not books and tuition), for the tax year, and 2. he did not claim himself as a dependent.
you two should have consulted on that before he filed, in order to choose wisely - the one to claim him as a dependent would be - logically - the greatest tax benefit to your family (you and him). Since he already filed, and claimed himself to make a change would necessitate an amended return and might - (might) trigger an audit.
I'd recommend leave it alone. and no you cannot claim him as a dependent if he claimed himself. (unless he files amended to remove his claim of being a dependent - you can NOT both claim him as a dependent.
regards
