hello,
if you could find a palpable lump, it may correlate with osteochondritis dissecans, considering his age. google this condition, look if it fits with your patient. if thats the case physio may not be a huge help. other possible chance could be an intraarticular loose body, from the fact other lig/menisci are normal. i agree with steve, for slim chances of referred pain. Other thing u could do, is a slr and add components such as hip add and ankle DF/PF and Inv/Eve sometimes if your sural nerve is involved or stretched u may get anke pain. does his knee give way, has he got an x-ray done, just to rule out. check his pirformis length. look for resisted knee extension in mid and outer ranges, to rule out osgood schlatters. there isn't a box standard answers, treat as u go. do update pls.