Minors taxes

Doesn't matter who files first.  Just be sure on his return he checks the box that says he can be claimed on someone else's return.  Under My Info.  

There is not a set rule as to who files first as long as he checks the box that says SOMEONE ELSE CAN CLAIM ME AS A DEPENDENT.  If he does not, and you try to claim him, then your return will be rejected. He would then have to Amend his return, and you would have to print and mail yours while his amendment was waiting to be processed.  

If you file your return first then you do avoid the worry of him not checking that box.