I’d advise not bringing up his stutter at all. I can’t really speak for your boyfriend or anyone, so this is my personal opinion. Even though my stutter is very mild, it’s still there, even though people tell me they don’t care, that nobody cares, that it is fine, uhh, in the end I DO care, I couldn’t give two shits that they think it’s no big deal. They don’t know how I feel and if someone told me that it’s sexy, I honestly don’t know how I’d feel ablut that, I guess awkward, irritated and baffeled at the same time, I’d think it was ridiculous, anyway it wouldn’t feel uplifting, I think. For a stutterer, it is best that you di not mention his/stutter at all, and I mean it not only with your words but also with your expression. Don’t tell them to calm down, to take a deep breath (You’re not helping, mom.), to slow down, etc. Anyways, don’t take this as an insult, this is my point of view, vent (to an extent), and an advice with no ill intent meant.