The calculation is not supported by the software for allocating a marketplace plan that is also used for the self-employed health insurance deduction. In this situation, the IRS does not prescribe that the allocation has to be 50/50 if you received advanced premium tax credit. You and your son can allocate the policy using whatever percentages you choose as long as they add up to 100, so you can allocate it to whichever one of you would derive the greatest tax credit.
