If your son qualifies as your dependent, then yes, you can claim him as your dependent even though he is on someone else's ACA policy.
You need to get your son's 1095-A and enter it on your return. Otherwise, the IRS will reject the e-file of your return because it will see the SS number on your return of someone who has a Marketplace policy that you did not account for.
If your son appears on someone else's return, you would need to enter his 1095-A on your return, then answer the questions in your interview about sharing him on another ACA policy.
If his dad made all the insurance payments and had responsibility for the policy, then you would allocate to yourself 0 (zero) percent of the policy, so that the PTC (if any) would go to him (I am assuming that you did not pay for your son's health insurance).
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