If you don't claim your son on your tax return because he is not your dependent and is filing his own return, you can still indicate that he is on your insurance policy (see screenshot).
If your son files his own return and reports his insurance, you can claim 50% of the premiums, SLCSP and APTC on your return (screenshot) and he can do the same on his return.
If his other parent claims him as a dependent, this would also be the case.
Here's an example:
"Child/Children on another tax return
If you share a policy with someone who is claimed on another tax return (usually a child), you can take the number of people on your tax return who are covered on that plan and divide it by the total number of people on the plan. This would be your percentage of premiums, SLCSP and advanced payment of the premium tax credit, unless you agreed on a different number beforehand."
You could also make your entries directly on the 1095-A in Forms mode (screenshot).
Edited 2/20/18 5:22 pm
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