Firstly i think your mom is wrong. Completely wrong. I have a mild stutter as well and sometimes it is worse i told my girlfriend at the time and now my wife. I stutter every now and then and now she considers it normal and doesn't really notice it every time either. Your boyfriend might be saving face by being nonchalant about it. I could see that if I did something wrong because i didn't know i would try to cover it up in some way? But i also feel like he doesn't get it. I think if you look at it from that point of view you'll be able to communicate your needs better. I can see that it is frustrating to have people do it all the time. Some of my oldest friends do it sometimes until this day however i dont feel like they intend to hurt me. It just what some fluent speaking people do, and i live with it. It's natural just like stuttering. Then there are those who do it all the time which can be really fucking annoying. I tend not to be around those people because i have better people in my life. If you like your boyfriend and you guys have said i love you and are in love he'll understand and make an effort. Perhaps explain that it is just something people who stutter deal with. Just don't put too much pressure on him if you know he cares about you, getting him to understand and laugh it off once in awhile could be healthy. I hope you know I'm on your side and that I can understand how you feel.