I work in orthopedics, and from your story it sounds like he may have a slight fracture. Often times fractures don't show up on x-rays right away, but if they re-x-ray the area a week later then the fracture can be seen. (this is because to heal a fracture the bodies cells clean out the bone around the fracture before laying down new bone) Also, often times places like urgent care, even pediatrics, miss fractures, they just don't look at x-rays as much or have the training that orthopedics or radiologists do. That being said kids are pretty good judges of their own injuries and avoiding activities that are painful until the injury heals. So he probably will be fine if you just give it some more time.
If you do want a 2nd opinion, or it's not getting better, I'd go to an orthopedic clinic. I'm not sure where you live but if you want recommendations I know good doctors from in Salt lake, bountiful, ogden, and Logan.
Good luck!