I believe that there is no PTC for taxpayers whose incomes are under the Federal Poverty Level because they likely qualify for Medicaid, therefore don't generally need to buy a Marketplace policy. In states with expanded Medicaid, the low income alone probably would have qualified your son for Medicaid. However, the eligibility requirements do vary from state to state. See https://www.healthcare.gov/medicaid-chip/ for the government's take on this situation.
