I would look into some type of fluency boot camp program. I was 26 when I attended my first one and it's been the only thing that was truly helped me. This program was a 3 week summer program of 5 hour days M-F where you learn techniques to help with stuttering. I was actually the oldest person in attendance with a bunch of younger kids so he's at the right age where this could really help. I've also tried going to a speech therapist and the main issue is that some therapist don't identify what is causing a person to stutter and how a certain technique helps. The whispering isn't the reason he's not stuttering; it's because he's keeping his vocal cords vibrating during the act of whispering. It's the same idea behind a person singing and when people say "Oh why don't you just sing to avoid stuttering." We produce sounds by our vocal cords vibrating and when the cords stop moving, that is when the stutter kicks in. And once that happens, we end up developing secondary ticks such as clapping or flinging your hands around to try to get the words out. I would actually end up puckering my lips and turn red like I was basically trying to use the bathroom. I'm now 31 and have managed to get my stutter under control. Some days are worse then others but I've successfully eliminated my ticks. If I'm struggling then I go back to what I learned in the fluency program to get me through. Hope this helps!