I wouldn't say it like that... Say what you mean. "I suspect that you are using again and would prefer that you stay away from Grandma's house." Whether he does or not is really between he and his grandmother, right? He may quack and squawk about how clean he is, but I would say "That may or may not be true for right now, but I have learned to trust my instincts, not listen to your words,...and my instincts tell me that you are using. Sorry."
I dunno... that's just me.