I believe it's natural for someone to stutter less or not at all when speaking in a second language, at first that is. Depends on the kind of stutter of course but I'm pretty convinced that as someone speaks more and more in that language, over a long period of time the amount of stuttering will increase to the point where it's the same as the native language. I think this is because someone's native language is acquired, and a second language is learned, so you could say you sort of cheat your way out of stuttering at first, because of the fact that you're still adapting to it. You say "naturally he's trying to be fluent in English" and that's exactly right, you don't learn a second language associating it with stuttering obviously, you think of yourself doing that fluently, and because it's new, that works. And that's actually subconsciously. There's a lot more going on behind speaking than just words out of your mouth, the stuttering just hasn't gotten a chance yet to present itself in that second language, it's there but it's asleep.