since your parents are obviously older, suggest the possibility of vaporizing. It seems to me most "adults" even the ones that have smoked in the past like new alternatives to taking a huge bong rip. vaporizing is just the trick! I just purchased my vapolution vaporizer from www.vapolution.com and it is fantastic! Maybe you have heard negative and positives about vaporizers, but the truth of the matter is that it is so healthy for you I almost thought it wasn't doing anything when I first tried it yesterday. This vaporizer is $125 shipped and has a heat setting control and is glass on glass rather than ceramic or metal heating elements.

Without having to buy anything I would just suggest waiting until he has had a few drinks in him (but not drunk) and just ask if he wants to "take a load off" or something like that. I dont know how your parents react to certain things but just propose it like you would any other friend or new acquaintence. Good luck! even more than drugs though, perhaps trying a family member / input based approach and try to help them as their son maybe. Switching from one habit to another is not necessarily going to produce outstanding results. Try having them work it out like humans ought to - with communication!