It sounds like you've made him realise he's got a fairly big personality flaw.
And to be fair to him, he's understood that it's not exactly a nice thing to be around. He's taking steps to change himself, he's catching himself when he does it and stopping himself. It's not going to be a quick process to train himself out of it though.
Someone I worked with in my late 20's pointed out to me that I finished pretty much every sentence with a nervous laugh, and that it made me sound like I didn't have any faith in what I was saying.
I had no idea I did it, and honestly, I hated them a little bit for telling me. From that point on I heard it every single time I did it, and it made me lose a lot of confidence in speaking. I successfully trained myself out of doing it, and I'm glad now that they told me, because if they hadn't I'd still be doing it, and people would take me less seriously than they do now.
Your boyfriend is in the same position. He's realised that a fundamental part of himself is something that other people dislike, and he probably hates himself a little every time he does it at the moment. But he is trying to fix it.
Whether he succeeds, noone knows. And you don't have to hang around to find out if you don't want to. I'd find someone correcting everything I said a massive turn off, and would probably have just dumped them without telling them why. But the fact he is at least trying to stop doing it would give me pause for thought.