Why would you correct him if you are trying to overcome a reputation as a corrector? Let it go. Sending else can straighten him out if it's important enough. Because it's embarrassing to find out you've been doing something like that repeatedly and in public, and that everyone knew about it, but no one told you. I regret not telling him at the time. It's like if you go to dinner and then to a party, and you get home to find you have lettuce stuck in your teeth. To me having had lettuce stuck in your teeth is way less embarrassing than the thought that it was there at the party and everyone knew it, and nobody said anything to you about it. I would much rather somebody tell me halfway through the party than not at all. So are we halfway through the party, or is it a week later?
It also occurs to me that somebody else might have told him in the interim.
I would say that enough months have gone by, hopefully someone has told him.
I think it's a month or two later. And you aren't even at the same party right now.
Now, if you called him and got him to speak French with you, and then you said, "oh, that verb should be this" and then moved right along with the conversation, I'd say you were totally OK.
But for you to bring it up now, after you've gone home from the party and it's months later, implies that you've been thinking about it an focusing on it. And it sends the message that the main reason you've contacting him is to correct him. Not fun to be on the receiving end of.
(I'm interested in your reason for telling him--is that why you correct people, do you think, or do you like the feeling of being right and being an expert? In a way, swooping in to 'save' someone is a bit *like* being an expert.)