If there was Advance credit (column C of the 1095-A), you DON'T look up the SLCSP for each family. You use what is on the 1095-A, and all three allocation numbers need to be the same.
If his income is over $11,770, it would be best to allocate 100% to the tax return that has the lowest poverty percentage (Line 5 of Form 8962). Unusually that is the kid. Otherwise, you probably want to allocate 100% on your tax return and 0% on his. Just be sure to enter the 1095-A on BOTH tax returns.
If there was NOT Advance credit, then you look up the SLCSP for each family. The 'link' in TurboTax should help explain how to manually do the math.
