Constant pain in a young athlete that doesn't go away with treatment does raise the suspicion of a more serious problem. What kind of tests have been done and what kind of treatment has he tried so far? If he hasn't been seen by a medical doctor, that's the first place to start.
Groin pain can be hard to diagnose accurately. Athletes who use kicking motions and who change directions quickly on the field are often prone to chronic groin pain. X-rays or other imaging studies may be able to show signs of stress at the pubis symphysis where the two pubic bones come together. Imaging studies would also show any signs of bone tumors.
Once a medical doctor has ruled out more serious bone or joint problems, a physical therapist might be able to help with other soft tissue or muscle injuries. Sometimes after an injury, the muscles must be given some help to get back to normal.