My Son Is Almost 15 And He Constantly Pops And

My son is almost 15 and he constantly pops and cracks when he walks, goes up stairs, etc. He does not complain of any pain but he can be heard walking anywhere in the house. Every article I have read talks about lack of protein or calcium. He drinks tons of milk and loves meat. He is 6'5 and about 240lbs but very active. Any ideas?