M.S.
I STRONGLY agree with the point you made in number 2. How would your husband like it if someone wouldn't stop tickling him if he asked? Your husband is bigger and stronger and that is almost an abusive type thing. It also doesn't allow your son to trust him. Stop should mean stop. Keep on him about that.
Bravo to you for noticing this relationship strain and trying to fix it. I hope you are able to get to the bottom of it.