A shared Marketplace policy is when there is more than one "tax family" on the same policy. If your son's mother claims your son as a dependent, then the policy is not shared. She will report the 1095-A on her tax return, and you will not report the 1095-A.
However, if you claim your son as a dependent, and your son's mother files a separate tax return, then it's a shared policy. If the latter scenario is the case, you will decide upon allocation percentages to figure the Premium Tax Credit. You can allocate in any way you would like, as long as the percentages sum to 100%. If you can not decide, then the default is to split the plan by the number of participants, so 50/50.
