What Causes Right Sided Pain Around Hips?

My son is 12 years old and today he said his right side bothers him occasionally. It is on his waist toward the side not middle back. Like if you put your hands on your hip with thumb pointing forward, the pain is were your fingers would be. He says it comes and goes usually when he is bending to the right side. He is going to bathroom on his regular schedule with no pains and his B.M s and urine are of normal colour. He says on scale of 1 to 10 the pain is about a 3. He says its not burning or stabing but rather a dull ache but goes away with repositioning himself. What would cause this? Also there is no fever or any other issues like dizziness or headache or vomiting.