Ipad: Limiting sound

I'm sure this has been asked before, but i cannot find an answer and if there isn't one then apple needs to do something about it.

I have an autistic son. Has his ipad mini 4 with all educational material and some entertainment. He's figured out how to put up the ipad into full volume. We lower it, few minutes go by and it's up again.

People have suggested setting a limit with headphones. He doesn't use headphones. Another suggestion was to run an application that does this, issues with them is if the application is shut off the problem persists.

Someone please help me find a way to lock the sound on the devise.