I'm busy this afternoon, so I don't have time to offer any cool links or research, but I do have some personal experience for you.

I played trombone for 7 years (6th grade - High school senior), and I absolutely loved it. It does just what you are hoping for - providing great oral sensory input. It improved my breathing, the sensation in my mouth (because I also had basically no sensation there when I was little), and it became a critical element in my sensory diet. It was the only class I enjoyed in high school, for the most part, and I only wish I still had a good opportunity to play. One of these days, I'll have to join one of those local community orchestras or something.

The buzzing, the vibrating, the noise, all of it, it all helps. I would absolutely recommend having kids with SPD pick up instruments even sooner than I did, like you are trying for your son. Good for you!