My 12 year old son has pain on his front thigh, we visited many doctors, no diagnosis

My 12 year old son has been complaining about pain on his front thigh (quadriceps), since he was 3 years old for both 2 legs. It happens to him after playing and doing activities like sports and after school. It happens to him at least once a week. and it lasts between 12 hours – 72 hours. some time he feels clicking on front quadriceps and he feel relief after clicking. Sometimes he says it is burning and stinging.
Sometimes he needs rest to get rid of the pain. sometimes it is saviour and he can’t walk even for a day and need pain killers. I could say it is always with him. This isssue has affected his running, sport and activities. He is always the last person in all sports activities. Mean he is too slow. we visited many doctors and different x rays and blood tests and many physios. But nothing has been diagnosed. I need help to know what to do and what is his problem. His health and life has been affected because of this issue and he has lost his confidence as he is always looser in all activity games.