You have 2 choices. 1) Accept that he fell while hopping, and assume that he bruised something and it hurts. He's favoring it by limping. He didn't tell you about it (which sometimes kids don't do if they are growing up and don't want Mom to overreact, and sometimes they don't if they were doing something else they shouldn't have been doing and don't want you to know). 2) Decide that your doctors did insufficient work, and see a specialist for another opinion, which could involve more tests (X-rays, MRI, etc.).
In neither case is worrying and being hurt an option. It doesn't help. In fact, is it at all possible that you are such a worrier that you don didn't tell you the whole story for fear of you overreacting? I ask this because I had a head injury as a child, sustained when I was riding my brother's bike. Because he had such a violent temper and because my parents were big punishers and poor listeners, I didn't tell anyone. I had a serious concussion but hid it, for fear of the anger and punishment. Dumb, I know. But I wasn't thinking straight because I had a head injury - vicious cycle.
What you can do to help your son is let him - and force him to - rest! He's trying to run, and he's re-injuring or straining something that already hurts! He's got inflammation in the hip or wherever the injury occurred, and it's not going to heal if he doesn't take it easy.
So decide if you are going to insist on him sitting more with his leg elevated and perhaps some ice on the hip area, or if you are going to go over the head of your doctors (which is okay) and get a second opinion as Doris Day recommends below.