You cannot claim your son as a dependent for half the year.
As he was under 19 at the end of 2017, you can claim him as a dependent if you provided more than half of his support in 2017.
He would have to file his own tax return if his wages (on form W-2) are more than $6,350 or his income reported on form 1099-MISC is more than $400. Below these levels, he can anyway choose to file a tax return to obtain a refund of federal tax withheld on form W-2. He has to indicate in the My Info section of TurboTax that he is claimed as a dependent by another taxpayer.
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