Ok, I have some ideas.
Idea One,
1. Make a room and remember the name of it.
2. Tell your brother to type in this command in free play: /jo room name (replace "room name" with your room name).
3. Pick a mod (setup, mods)
4. Say this command when he gets in the room: /op Your brother's name (Replace "Your brother's name" with your brothers account ID) This will give him permission to change the game rules.
5. HAVE FUN
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Idea Two,
Go into a public room.