I would show him his leg drive and intent. Jimenez does a good job in both essential areas and I think would be a good spot to start with a ten year old.
But that isn't set in stone. A lot depends on the kid himself and seeing him pitch. If he is good in those areas I would probably try to focus on separation, since that is an area that most pitchers at every level struggle with.
If I was going to show the kid his mechanics in particular, I would definanteky would want to show him some slow motion and full speed so he can see the areas I'm talking about. That way I could engage all three learning areas. Visual, hearing, and doing
And also his head. Making sure his head found the target early and locked on it.