What race from the PHB has he chosen?
If you’re going full on homebrew and not using the PHB, it might be pretty rough as a new DM. You won’t have baseline rules to refer to and you’ll be making up a lot during game play.
That’s not a bad thing. Just challenging for a new DM.
For a new DM, I always recommend The Starter Set from Wizards of the Coast. This has easy to read rules, pregenerated characters so you can start right away and a complete campaign which is really fun and has lots of side quests and hooks to keep the game going for years.
If you’re dead set on playing your own homebrew world, that’s great. But be prepared for a lot of arguments from that Player, because you don’t have rules to control him.
Good luck!
Let us know how it goes 😃
I always recommend The Starter Set from Wizards of the Coast. This has easy to read rules, pregenerated characters so you can start right away and a complete campaign which is really fun and has lots of side quests and hooks to keep the game going for years.
But to answer your question, put the responsibility on the players. “You return with the child. Now what do you do?”
Have a few plot hooks for one shot encounters/sessions so you have time to expand on their part of the world, but the world can grow as the characters do.
Did you watch Stranger Things?
Season one was all about their neighborhood where the homes are. Season two added the larger town/downtown. Season three added the mask and neighboring towns.
That’s an easy way to build your world. Hometown. Larger city. Then state. Then country. Then…
Let it develop over time as they play.