Answer:
Because the home is not Dad's principal dwelling, Dad would not have a right to rescind the transaction. Son would have a rescission right, assuming that the transaction or event requires it. But note that there never has been a regulatory requirement that the consumer sign the Notice of Right to Rescind.
If, however, your bank's policy or procedures call for such a signature (the requirement is often imposed by secondary market investors) and the form you use provides lines for consumers' signatures acknowledging receipt of the notice, you should obtain the son's signature there.