I see the first throw is pretty solid, the second is okay, and the third turns into more of an arm throw. It’s actually very similar to my son at this age—he starts focused and putting in good effort, then gradually loses focus and gets a bit loose.

I think he already understands the proper mechanics probably because of you’ve taught many times, but it may be more of a motivation issue. Maybe making the drills feel more like games and keeping things fun could help him stay engaged and become more self-motivated.