#If you have access to published adventures, run them instead

I know he probably wants to create his own world, make his own encounters, do his own thing, but clearly he is having problems with that. I would recommend him to run a premade adventure first. It helps to get the grasp on mechanics, balancing and everything else.

The Lost Mines of Phandelver is great for that, it comes on the Starter Set and is very clear on what both the DM and players need to know at each point of the game. While guiding the DM nicely, it still leaves some holes for the DM to fill, so he won't be just reading you guys a book.

Two DMs: The "good DM" (story teller) and the "evil DM" (combat DM)

This is a system I have seen on a streamer that I follow. It consists on having two DM - one who creates all the plot and tells the story, and the other creates the encounters and controls the monsters in combat. I've done this once in basically your situation - a new DM that wanted to tell his story but didn't know how to properly control the monsters or create balanced encounters. It would get messy sometimes, mainly when we forgot to communicate decently before a session, but he learned quickly about the combat and was able to tell his story. In the end he even asked me to play the final boss and did a great job on it.

#Show him DM tools

DMs, let's be honest - except for very important characters, encounters and dungeons, we usually run some kind of Random X generator. At least I do, many (most) of the DMs I know do, mainly when we want to run some dungeon crawling, for example.

Even if he does not use the random generated encounter, he might use it as a base for what a balanced encounter for that party looks like. I'm not linking these tools because I'm not sure it's allowed here, but you can find them easily.