Have you checked the lumbar spine? I notice you didn't mention if he's c/o LBP which i then presume he's not but increasing clumsiness, lateral thigh pain (as opposed to groin-hip pain) and anything bilaterally is NOT good in kids. Check him out to clear a spondylolysthesis and get Lx Xrays (i assume the Xrays you have are hips) if needed (plain view AND in flexion/extension). Also have you done a neurological examination of the LL's (reflexes, sensation, strength) as he's got bilateral Sx, it's really important to clear any other really nasty red flags (like a serious neuro issues like muscular dystrophy etc). Make sure there's no scary stuff going on...
So basically I'd suggest get the all clear on the lumbar spine and neuro things first and then continue with the core strength. Fitballs are great as is sport-specific training if he's up to that level and/or interested in a particular sport.
Let us know how you go.