short answer = get professional help
long answer = what are your goals here? if its to notify him that hes a crackhead, then tell him he's a crackhead and fucking annoying to be around
if you dont want to see him kill himself then tell him, dont come around me on that shit. if you do, im leaving
other than that, you probably dont have the pull to 'force' him into an intervention. I dont mean physically, i mean im guessing you dont pay his bills so 'fuck you if i dont want to go i aint going'
im not a beleiver in the whole 'tough love' you got one shot bullshit. Id tell him the truth, drugs are going to kill you, good luck, you want to hang out, lets go, but not while your high.
be there for him, hopefully he'll pull his head out of his ass before he dies.
oh and rehabs are not useless, but expensive, hope hes got insurance.