How to prevent lock/unlock on ipad to be changed in settings?

My son uses my ipad for gaming, and sometimes for school as well. He’s been struggling with screentime-addiction so the time is restricted in the settings. But he changes the automatically “lock” settings to never, so it’s constantly unlocked, every time he gets my ipad in his hands. I tried to change a bunch of things to prevent this from happening, but nothing works. Whch means he cn then open the ipad at his will, when I’m to the store, work, asleep… Do I really have to put my ipad behind a real lock? I’m looking for a solution to prevent him from changing this setting. Please tell me it can be done. Thank you.