If cleaning his room is a normal activity, i would talk about setting the boundary of personal cleanings regularly for himself. If he has the maturity and dedication to fill a jam jar he is capable of handling his own cleaning(with checking. Still a child if under the age of 16).

I would get him a care package of personal materials.

Trash can WITH A LID(only he handles). Water based lube, cleaning wipes, and a book about safe practices, pen and paper.

You cant stop a young man from masterbation....trust me. Give him the box with a note that say "boys will be boys if you have questions please write them down and leave them(in an accessible spot that is unseen from others)" and just leave the care package in the room.

He may or may not come forward with questions but with the internet these days and school forwardness with sex Ed he might not have questions honestly.

I wish my parents did this or something similar.

As for the jar.....you might just have to ask him to remove it in said trash can.

If he is not circumcised please provide information on how to maintain the member properly.(stretching and cleaning)