Adult stutterer here. I attended treatment with an SLP through my school district from 1st grade through 10th grade. If you can, PLEASE encourage him to keep going to therapy. I went through the same thing, and it was just a phase. If he can get to a point where he sees some results from the strategies he is being taught, he will be more receptive to keep using them. Regarding friends, he WILL have to deal kids or situations that aren't great. It happens, and some teenage kids suck. That being said, his TRUE friends will be there for him and accept him for who he is. Professionally, it will only hold him back if he lets it. I have a Mech Eng. degree and am a lead Design Engineer at my company. My stutter has never held me back from professional success because I haven't let it, and nor should he. Also, during college I was in Air Force ROTC and it didn't hold me back one bit.