I know you have the best intentions, and if your SO wants to change it’s great. But please talk to him first. I want to share my own perspective as a stutterer: I hate it when people try to find some kind of cure for me. Especially when my ex-boyfriend found some therapist for me when I never asked him to - it felt like he wanted to change me, like I was damaged and needed to be fixed. Personally I’ve made peace with my stutter and I want people to accept me with it and love me for who I am. So, obviously, be supportive with your SO but try to talk to him first and get his opinion. And remember that speech therapy helps a lot (adults too!), but it probably won’t cure stutter 100%.