I'm glad he is ok. I have a story about a girl, not a boy, and she was 12, not 5. My dd is extremely intelligent but sometimes she just doesn't think. She does trampoline and tumbling. We were in a hotel the night before States last year. We had a 1 bedroom, so she was in the bedroom watching TV and I was in living room. I was just getting ready to tell her that it was time to go to practice at the gym where they would be competing.
I heard a little thud. Suddenly I hear this strange sound almost like a goat bleating "Ma-ma". By the time I got up she had opened the door. She had been flipping on the bed and slammed into the window. The window didn't break, but the marbles sills were very sharp. Blood was pooling at her feet and her shins were sliced open. 5 hours later after, 911, ER visit from hades, many stitches, IV, etc we got back to the room.
Next morning as I am packing to leave, I look out of that window. We were on the 3rd floor, and as I looked out all I saw was pavement and cars below. That day I thanked God for not allowing that window to break and for that marble window sill that stopped her momentum.
I'm so glad he is ok, but be very careful. You would think they learn their lesson, but 3 days later doing a follow up visit with dd's doctor at home (on the day we were supposed to be leaving for our vacation to Domincan Republic) and he asked her if she had learned anything from it. She said, "Yes. Tumble AWAY from the window."

So I would take precautions as others have suggested, such as blocking the windows, etc.