Because he made less than $12,400 he's not required to file. However, if any taxes were withheld from that pay, then he should file to get those withheld taxes refunded to him. Take note that only the amount in box 2 of the W-2 will be refunded. Box 4 and 6 are never refunded.
If your son chooses to file to get a refund of any withheld tax, then he "must" check the box for "I can be claimed on someone else's tax return". If he does not select that box, then it will cause "you" problems with filing your tax return, assuming you are claiming him as a dependent.
