Thought I'd add that my son also uses the MarcPro between innings while pitching. It's his experience that it keeps his arm loose and he can step on the mound without really needing more than one or two warm up pitches.
The discussion on the MarcPro is interesting.
Kyle mentioned it for recovery. It sounds like your son is doing it for general looseness?
For pitchers, is MarcPro used on the shoulder? Elsewhere?