I'd tell him straight - pull a stunt like that again and you're out.
Pulling the whole band down like that is out of order.
+1
By the way, he's gonna be thinking "how the hell could they think it was sensible starting without me?". From his point of view, that's valid.... so you also need to address that in order to keep friendly relations going. I'd suggest pointing out that the only other option was to pull out altogether and, among other things, you decided to "protect him from being entirely responsible for that" by covering for him.
I slightly p1ssed off my drummer once by starting an encore without him - he'd gone off to get a drink before last orders. As I saw it I had a choice, we played an encore or we waited for him and didn't (an encore would have gone over time then - I was very particualr about not p1ssing off landlords). So I announced what we were doing, and told the barmaid to serve the drummer asap, we all had a good laugh (audience and barmaid included) except the drummer...
After the gig he tried to tear me off a strip, I proceeded to start tearing him off a strip and, much to my surprise the keyboard-player and bassist weighed in as well. When you're gigging you're gigging, takes precedence over everything else, organise yourself a bit better. He didn't do it again...
and it was forgotten quickly and we all remained friendly.