My son has pain in his right knee from one year. Why?

Hello doctor,

My son who is turning 4 in two months, has been complaining about pain in his knee for over a year now. His pediatrician told us that it was growing pains and is normal. However, it has been there for too long and it is always just the right knee. He cries at different times in the day about his pain, mostly at night and after school. He holds on to his leg right under his chin, cries, and does not let us touch him. Some nights he cries himself to sleep from the pain. There is no swelling, bruises, redness, or anything in his knee that looks or feels abnormal. What could it possibly be? Which specialized doctor should I take him to and what tests should I expect him to do?