how to block setting changes during downtime?

If Screen Time passcode is enabled he should not be able to change the time zone. The time is forced to Automatic when there is a Screen Time passcode.

Clever. Encourage him, then. Maybe keep the batteries in his you-should-not-be-reading reading flashlight charged, too.

For efforts to suppress that curiosity and cleverness, you’re headed toward profiles and device provisioning.

Longer-term... Expect to lose this battle, too. Kids can and will evade every block, if they’re inclined to do so. They’re clever, adaptable, observant, persistent, and an intractable foe, if you choose to make them into that foe. And kids will learn from us even what we don’t intend to teach them, both parents and educators. And what they don’t learn of us and our limits and our foible, they’ll learn from friends and forums.