He was 5 years old and he didn’t know the president had been shot. What he saw was Booth jumping from the balcony into the president’s viewing box. He thought the guy had fallen. (Booth broke his leg in the process.)
It was never proven that Booth broke his leg when jumping on stage. He was able to run off stage and mount his horse on that leg. However, his horse fell on him later that night and only after that was there any evidence of an injury.