For the months that your son was on your policy, you can allocate the amounts in any way you both agree. This means you can allocate 100% to yourself, or you can split it anyway you choose. If you both agree, the allocation can be anywhere from 0% to 100%.
If you cannot agree, and Advance credit was received (column C on the 1095-A), then it is divided by the number of people on the insurance policy. For example, if the insurance policy covers 4 people, each person is allocated 25%.
See this TurboTax tips article for more information about Form 1095-A.
See also this IRS article for more information about the Premium Tax Credit.