What you are talking about is patellar luxation (AKA luxating patellas)
Doing a search of the site will bring up lots of info on the condition.
If he knocked the patella (kneecap) out of place in the fall that will be extremely painful. It doesn't mean it will happen again though, that is down to anatomy.
Last edited by Wicked Pixie; 07-15-2016 at 04:09 PM.