Had My 12 Year Olds Left Knee Checked Out And

Had my 12 year olds left knee checked out and was told he had a sliding knee cap (patella disorder) He began physacle therapy and has had a few sessions along with excersises to strengthen some key muscles to help with this problem. He's also a hockey player and has sat out several practices. He participated this evening in his first practice again. He said he had no knee sliding or pain following skating but now has a grinding/poping sound from both knees. Wondering how typical something like this is. He did not get an X-ray during diagnosis for patella wondering now if he should have one taken to rule anything else out.