So growing up talking to girls I noticed when I stuttered, I always observed their reaction. The worst reactions were those that turned away or tried to ignore the fact that i stuttered. When they stayed engaed despite my stutter, it helped. Even if they squinted their eyes, as long as they stayed engaged until I finished. But it also depends how bad it is. If it's very subtle like mine, sometimes we stutter just a bit and we dont notice it. Other people will, so if its a subtle one like "E-E-Especially bla bla bla..." I definitely wouldnt make a deal about it. As far as brining it up, it wouldnt be that bad to bring it up depending how close your relationship is. The only safe way I can think of brining it up, is asking if they had a stutter as a kid. That will show there are small noticeable signs but not big enough to be sure its even a stutter. Or wait until he brings it up. You can make him feel better by sharing a struggle you dealt with growing up in your childhood. Hope this helps