The best way I can think to bring it up as a topic without coming off as insensitive or inorganic is to bring up the movie " the king's speech" it's all about the British king during ww2 coping with his stutter I feel like talking about a movie like that would give him the opportunity if he wanted to talk about his stutter but would also give him alternative topics to talk about if he felt uncomfortable about talking about his stutter Btw I feel if he's comfortable talking to you on the phone he's probably comfortable enough with his stutter around you to talk about it because most if not all stutters stutter much worse over the phone I personally avoid talking on the phone as much as possible as do a lot of people who stutter