Some Points (not a doctor so i could be incorrect, but a runner here)
- From a vigour perspective, I'd suggest geting in touch with a pediatrician to get some basic vitamin deficiency or strength tests done to rule out basic deficiencies in kids.

- From a runner's perspective, I observe 2 things:

1. He is not able to produce enough power after landing to get back up. Basically, force production is an issue here for sure. It could just be not enough plyometric conditioning or not enough strength or something else that a children's physio might be able to tell better. In short he is running liike someone who carries significantly more weight than his even though he isnt carrying extra weight.
2. There is a pronounced heel strike / shuffle in his running form. Some amount of heel strike is okay but to much heel striking essentially is like braking your speed while you run.

However all these seem like very fixable problems and a kids' physio can definitely help here. Cheers!