Well, I would make sure that you want to stay buds with this guy. If he's hanging out with a buncha of pousers, a group you don't particularly like, stay clear for a while. If spending time with him means spending time with the group you don't like, then I'd say let it go.
If, however, you can spend some time one on one with him, do some reasearch on mj and expose some of the lies to him. He needs to get that D.A.R.E shit out of his head. Let him make up his own mind about it, after hearing the truth. If all is cool between you, then just do the shit you normally would. Tell him "I won't try to convert you, if you will let make my own choices." If the nagging starts up again, it may be time to cut the strings.

Peace