Yes and no... That was one of my parents main complaints, I think. It is impossible for a physical mouth to keep up with the speed of thought but I certainly tried.
I still speak very quickly on topics of interest when I don't pay attention to it, also when I've had a drink or two (Hurray for speed slurring!).
Suggestions to help your son:
Make a recording of him talking his normal speed and then very carefully talking at a humanly understandable speed so that HE can hear the difference.
If you are even slightly bilingual try to get him to say it in his less fluent language.
Tell him to pretend he is giving a presentation at school and the "presentation" part is where the grade is coming from.
Explain that you really do have trouble understanding him at that speed, then mimic him so he can understand.
If all else fails, tell him you are are absolutely flat out stupid and unless you can understand him clearly then he'll have to keep repeating it until you can! lol slightly joking...
