"...what is the correct protocol when I see him drinking?..."
As you can see from the responses here, there is NO 'correct' protocol (besides which, isn't 'correct protol' a bit redundant, anyway? LOLOL); in fact, for me, I don't see any 'protocol' at all.
"...Do I leave and say I do not approve, and potentially make him more angry/guilty/etc. or do I stay, essentially reinforcing that his behavior is okay because I tolerate it..."
As many of the responses seem to mirror, you seem to see this as an 'either/or' proposition. For me, I guess, I see it as.....it depends on where you may be when you do see him...this could be on the street, at your place, at his place, in a club.....lots of different places, which could mean lots of different reactions, and also it depends on your comfort level of being around folks who are drinking, and your recovery.
I believe you stated that your friend has said to you that he is concerned with his behavior and that he wants to be sober, therefore I might let this be my guide. Depending on how 'into' his drinking he may be at the time of your 'connection' might guide you on whether to speak to him at length, but for me, just turning and leaving might just reinforce a feeling that nobody cares (for him, or what happens to him) any more. A few choice words with a promise that you're there for him if he wants to put down the drink and get sober; that you're willing to help all you can.
As for getting between an active alcoholic and his drink, no, I would not do that (as Omega Man has suggested); not due to any 'safety' issues. It's just NOT my business, and I certainly wouldn't let anyone put me in that position either. If someone wants to drink, to me, that's their business, definitely not mine to control; my recovery is strong enough to allow others to be however they want to be at any given time, and if that means they're drinking, then I guess that's what they'll do.
In your friend's more sober moments, if I were in your shoes, I might tell him that if he were to be at my house, there's a 'no drinking rule' there. I might also tell him that I can't be around someone while they're drinking, therefore I couldn't be at his house while he's drinking, and this might include any other places (clubs obviously excepted) where we might meet; I might have to leave him to his drinking as I just can't be around it, at least not at this time in my recovery (unless of course at this time in your recovery it doesn't bother you to be around folks who are drinking).
All this can, and probably should, be done at a time when your friend is not drinking, and so hopefully he'll remember (mebbe?) at times when he is....talk to your friend; you might be surprised by his response, and you may be able to help.....who knows....?
NoelleR
P.S. You know the ole 'love and tolerance' saying. For me, there are some things that are just intollerable. When it comes to folks drinking, it just comes down to what am I willing to tolerate; it's totally subjective.