My 5-yr-old Was Playing On Pull-up Bar, Jumping From A

My 5-yr-old was playing on pull-up bar, jumping from a tall stool up to the bar. His hands slipped and he fell on his back. I did not see him fall. I imagine he had the wind quite knocked out of him, and he cried hard, naturally, but immediately said his belly hurt, not his back. He says two hours later that it still hurts, a 10 on the scale. (We've never used that scale before, but I think he understands what I mean.) what could this be and how urgent might it be?