Have you ever taken a fishing hook to the nipple by an angry father?
Personally, I'd just tell him that you know he used to do it, and that you both do it, and want to know if he still does do it. If positive answers come, ask if he's interested in some "herbal bonding".
I wouldn't just whip out a j seeing as how he may not be able to smoke because his job drug tests him or simply because he doesn't want to re-enter something that was hard for him to break out of and I wouldn't want to be gone with my dad completely sober next to me.