My sons father passed away and still owes back pay as well as arrears and I understand that what he owes will most likely be forgiven however my son is 20. Would he still qualify for social security benefits or is there an age limit?

The child support arrears are a debt of his estate. If he has any assets, and the estate is probated, you (not your son) can make a claim against the estate.

As for social security, you can check with the SSA, but I believe such benefits are only for minors.