The legal limit in my state is .08. But to get there, depending on height and body weight, you'd have to down those 5 shots in about 2 hours on average. Further, my state has a law stating that bars and such are liable for drunks in their establishment if they drive home, IDK if hes drinking at your house, or in a bar somewhere, but thats something to look into if hes in a public place drinking (The bar will literally have a bouncer confiscate your keys if they think you're too drunk, or call the cops if they dont have a bouncer). Finally, in my state (Again, alot of this depends on the laws in your state), DUI is a $5000 ticket, so not something to take lightly.
Now, since you said hes just a friend, America being what it is, someone, or two or three, I didn't read all the posts, it really is his life and theres only so much you can do to exert any sort of control over him. You can call the cops and give them his license plate, but if you can't even tell hes drunk, theres a decent chance a cop wouldn't either, and refusing a breathalyzer isn't a crime in most states, its a way of avoiding the cop discovering you've been drinking at all. You can call em anyway if you're really concerned, thats what they're there for, but you might alienate his friendship in the process. It really boils down to what you want to do or care about.
My personal suggestion is to let him have his own life in his own hands. I see drunks on the road all the time (Particularly during summer, and on Fri/Sat nights), but I don't call the cops on em because if they end up in a crash and kill someone else, I figure thats their choice to live a life like that, sad as it might be, trying to "help" them by sticking my nose into it is just as likely to get my nose bent out of shape for my trouble, so No Thanks. Maybe they'll be really lucky and have an accident that only involves them and they'll learn a greater lesson then I or anyone else could ever teach them.
Thanks for reading if you have.