In person is the way to go. And then making in person contacts to the overkill stage. It will be arduous at first, but it's worth it. Desensitization therapy works. It's CBT, and that has a good track record. Also, do everything in his power to keep up his self-esteem. Going to the gym is the best advice for almost anyone! Dress well. Confidence lessens stuttering for most. Do not catastrophize it when it gets bad. That will lead to depression. Find a way to joke about it. I find for many if we learn to slow down (pausing for me) it greatly reduces it. Like how Joe Biden practiced with pauses and cadences. Our brains are like weaker engines. Therefore, if we talk slower there is less turbulence.