You're definitely not an asshole, just misguided in my opinion. You're trying to help and you actually seem to care, which is more than most people I've met in my life. Stuttering can be improved. Yes, it is worst in some but with proper therapy and more deliberate practice it can definitely improve. If I were you, I would encourage him to practice speaking with you. Tell him that you will not judge his stutter and encourage him to be comfortable around you. There is no cure for stuttering, but learning to be more comfortable with it goes a long way towards one's mental health. If you can help him with that, you will be doing him a tremendous favor.