I doubt Toastmastets would help with his stutter. Stuttering is neurological in origin. Physical actions like therapy can mitigate it, but they don't get rid of it. Toastmasters is neither, it's for people that are afraid or bad at public speaking. Most stutterers are afraid of speaking because they stutter, so it wouldn't help much for most people. Depending on how mild it is, it may not be a big deal in that role either. If it is, I would suggest finding someone else unless he's actually up for it. I don't think it's off limits, but I'm sure he's thought about a lot of this already.