First, are you on probation or being drug tested?

Second, something about his offer makes me a little leery, but I think that's because I'm a mom. I mean, is there any possibility that this guy's a pedophile and is interested in you in that way? I'm always suspicious when grown men (or "old guys," as you term this one) are interested in cultivating friendships with adolescent young men and offering them places in which to drink or get stoned, but perhaps his motivations are innocent. Let's hope so.

If you think he's totally safe and above board, I think the best approach is just a direct one. Talk to your neighbor. Tell him you're a cannabis fan but that your friends are losers and your girlfirend is opposed. Ask him if his offer was genuine. If you do decide to smoke at his place with him there, don't get too stoned the first few times so you can make sure you're safe. And make sure you're not the one who's always supplying the weed, if that turns out to be the other part of his motivation. Also, I'm sure you're already planning to do this, but be very discreet about what you're doing and who you talk to about it. If he does turn out to have honorable intentions, you don't want to put him or your new friendship at any risk.