Knee Injury

I need some help guys my 17 yr old son has dislocated his knee cap twice this baseball season…I’ll start at the begining…3 months ago he was blocking a ball and his knee cap became dislocated…no tissue damage… well on saturday it happened again…the second time he was not wearing his brace… and there was no tissue or cartilidge dammage according to the mri…

what i’m looking for is any type of streanthining exercise that may help prevent this in the future…
Bulldogg

Leg Extensions