I concur with what has already been said. Just be like, "Hey, your weed was okay, but I don't really like getting a headache when I smoke, or having to pick out stems and seeds for 20 minutes before I toke. Do you have anything that's higher quality?" If he says anything like "Man, what are you talking about, this is dank" or "this shit is toppa da line!" or "no, this is all I have." Just take your business elsewhere. You don't need this kid, don't let him think that you do, you see?

Hope it goes well for you, and I hope that you get a great chronic hookup man.