There is no reason that you should not allocate the higher portion to your son. The 1095-A can be allocated for the best benefit of those on the policy without any trouble from the IRS.
All percentages need to be the same. So if you allocate 100% to him, he also claims 100% of the Advance credit.
It is legal, BUT you are also required to report changes to the Marketplace. So you need to tell the Marketplace that he is no longer your dependent. Then he will get his own 1095-A next year.
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