If he doesn't personally want to work on it, and if it's not a big deal to him. Then it should be fine. There's no cure, and a lot of people just choose to come to the conclusion that it's never going to change so I'll just live with it. And that's okay. If it isn't a big deal to him, then it shouldn't be for you either. It's just something that makes him different. If techniques help him, and he chooses to not use them, he's obviously made the choice in his head that he's okay with stuttering. It's your choice whether you're okay with that or not. And if you truly love him, his stutter shouldn't bother you at all.