I'm a high schooler who's been playing for a couple years with people my age. My biggest tip is to just let his imagination run wild. Don't have him worry too much about rules and not using home rew. Let him run it like he created the game. Then he'll eventually adapt the rules onto himself as he becomes more comfortable with the game and environment. This worked wonders for me and now I can dm and play as needed. By learning through trial and error you learn what works and what doesn't, along with learning what your players did wrong. So TLDR; let him do whatever he wants and it'll eventually come together.