irs rejected 17yr old sons' return because we claimed him on our taxes. T. tx says we must physically mail his return. Should I check box saying 'someone can claim him"?

Yes. Check the box saying someone can claim him since that makes it a correct return. 

His return needs to be marked that you claimed him as a dependent. Then you can resubmit the e-file. You do not need to mail once you have made the correction.

Did you find where to check it on his return?  See the screenshot I posted at the end of this thread

Why can't I mark myself as being able to be claimed on someone else's taxes?