It can be whatever you want it to be. If to makes more sense for you to claim the premiums paid on your tax return, then you can do it that way. Your son just will need to indicate that he had health insurance. You can report the percentages however you want on each return.
@TaxGuyBill has a great answer to this that he shared with others in similar situations as yours:
"If you do allocate, you enter the full amount of the 1095-A on all three tax returns. You would fill out the allocation part of 8962 and only 33% of the 1095-A (or whatever other agree upon percentage) will flow to the actual 8962.
However, in most cases it will save the most money to allocate 100% of the premiums to one tax return (if they all agree to it). It will save the most money to allocate it to the person with the lowest FPL and income. In this case, it sounds like they are all one-family tax returns, so allocating 100% to the lowest income will result in the best Premium Tax Credit.
EDIT: If you allocate 100% to one tax return, the other return (with 0%) still needs to have Form 8962 filled out, with 0% in the allocation field.
As for the mother's name on the 1095-A, that SHOULD be okay. The dependent's Social Security number is still on the 1095-A, and the IRS computers SHOULD be programmed for that."
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