I was a junior who everyone thought would go pro. At 19 I hung the clubs up after watching my father have a heated argument on the course over a game.

At 39 I dusted them off and started playing again.
Let him make the game and practice serious not you, if I shot 4 under dad thought I should have been 6 under. No matter what I did it was never good enough, what a relief when girls and motorcycles came along.

But the short game is where results occur and the fastest way to get your score down.
Dont change his normal routine ie sleep food etc to close to a tournament.
What helped me alot was stress the night before and I couldn't sleep so turned up tired and feeling terrible. That defeated me before the first tee. If only I'd been told earlier that no one sleeps before a big game, just try to rest the body wether you sleep or not doesn't matter.
Change the game a bit, no woods off the tee every second or third round. Make him play out of his comfort zone and shots from positions he's not used to.( make the young mind think rather than just swing)
But he really needs to hit the golf course more often.

Not really answering your questions sorry, but theres no quick fix. Get the pro's to draw up a program for him to follow including exercises.

Good luck