Here's what you do:

Get some fake weed, doesn't matter what it is, as long as it won't kill him.

Tell EVERYBODY except him that hangs out at this spot that it's fake weed.

Tell him that you have a treat for him because you like him (or some other bullshit...)

Hand him a bowl or joint of aforesaid fake weed.

Call it "Royal White Lights" (a bullshit cannabis strain name that I just made up, but it sounds like some dank shit)

If he asks, tell him its a hybrid-hydro chronic that is composed of White Widow, Northern Lights, etc. Tell him that it has special properties that do not give it a strong scent or hairs, a truly rare find. And that it is potent shit. Give him a bunch of facts that he won't know about, but it make it seem like you know your shit.

Watch him get blazed off his ass, then make fun of him and/or make him cry.

Edit: That is, if you really hate him, I'd probably do it, but then again, I am an asshole.