Do I add my underage son w2 to my tax return

You would not add your son's W-2 to your tax return.  For 2016, Dependents who are not 65 or older or blind, who have earned income more than $6,300, must file their own return.  If your son earned less than $6,300 in 2016, and is eligible to be claimed as a dependent,  he does not have to file a return, but may wish to do so to recover any withheld income taxes. In addition, If your son still meets the requirements to be eligible to be claimed as your dependent, and is only filing his own tax return to get back any taxes that were withheld from his pay, then he can file his tax return and indicate that he can be claimed as a dependent by someone else, and you can file your return and claim him as a dependent. Please see the following TurboTax FAQ for the eligibility requirements that determine whether a person can be claimed as a dependent on your federal tax return:

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Your son will file his own tax return. When you complete it for him on his behalf, make sure you select the option on his return for "I can be claimed on someone else's return". If you don't select that, then you can't claim him as your dependent.