Can MRI detect injury on the hip for a 8yr who played soccer for 2 yr. What can it be? After he stopped playing soccer he has on & off pain in the knee

Yes. Before jumping to the big gun, have you taken him to a qualified pediatric orthopedic surgeon for a thorough evaluation? Have a professional "lay hands" on him, so to speak. i.e. to physically examine his hips knees etc? Yes, knee pain can be referred from the hip. Also taking a careful history of the events leading to the pain also can tell a lot. I would take these basic steps, then MRI.