Son was riding today and this happened... Not sure what happened,.but more importantly, how do I fix it lol

It does slide back in but bearing is destroyed... Do I have to split the case?

 

Never done one, Kawasakis seem to be unpopular in the UK, since they stopped making the KLF300 they have lost dealers. Anyway, there seems no oil on the components so I would guess the gearbox has run dry and the whole gearbox will need a rebuild. Unlikely you could get the old bearing race out and a new one in without removing the gearbox cover anyway. Looks like there is room to do it with the engine in the frame. You might need to remove the belt as it may be pulling at the input shaft.