Have you considered fat pad syndrome or tibial nerve entrapment? Check out David Butler as a resource for checking the tibial nerve (may be caught within or behind the tight gastroc). Also check his cycling technique, is he staying in plantar flexion throughout his downstroke? He should be starting his upstroke with his hamstrings by keeping his heel lower than his toes. A lot of people have the bad habit of keeping their toes pointed throughout their cycle stroke.
Best of luck with the differential diagnosis.
K