I don't know if this might sound rude or annoying, but can I suggest a scene? What if the father comes home from work being stressed, tired, and has severe back pain and the son offers to give him a back massage which the father accepts but what the son did not expect was that the father would undress completely and lie on his stomach leaving his ass completely exposed and the son would begin to massage his back and this is where the protagonist would decide if his hands would go down to massage his father's ass (which would lead to a scene of the father being bottom) or give him a normal massage until the father turns around and wants his abdomen to be massaged down to his cock (which would lead to a scene of the father being top), sorry if you don't like the idea
Naked Father Reference Image