Some good Points Alophysio..an assesment would be the best way to find out !!
- maybe the muscles are in a protective spasm for something like a stress fracture or labral tear? "On Both Arms?"
- maybe he has a poor stabilisation strategy and the muscles are being used as stabilisers instead of movers"Possible"
- maybe he has a motor pattern that overuses these muscles instead of other muscles 'definately possible"
- maybe he has a neck problem causing scalenes, upper traps, levator scap, etc to overwork 'definately possible"
- maybe the neck problem has a neural component "Trigger Points so close to the neck will definately cause neural problems"
- maybe he really does have a Wikipedia reference-linkrotator cuff tear - rare in a 21y.o.
Knowing if mhewitt does weights regularly or has done in the past would be usefull.Or any other Exercise regime that could have bad form"

Just my thoughts
Cheers
Dan