Witnesses to a will [usually] cannot be beneficiaries of that will or heirs to the estate. So if your conclusion rests upon the witness to a will being the son of the person who wrote it, your conclusion probably needs some revision and you need to check the contemporary state statute to determine who could act as a valid witness.
Witnesses can be relatives--just [usually] not beneficiaries or heirs. A brother or brother-in-law could witness a will as long as they did not stand to inherit. In some states they could not witness if they were an heir or beneficiary.