Without going into too many details, this is all technique and muscle memory, just like any fundamental in any sport. In practice, I would watch his technique on XP’s and make sure he is doing the same on FGs. Correct steps, lining up the kick, plant foot firm and next to the ball spot, same delivery and follow through with kicking leg, head down, and there is more with upper body getting square on ball momentum forward and turning through the kick. You master that muscle memory with XPs and then translate to FG attempts. While teaching all that, build confidence and encourage while correcting what goes wrong and what you can do better. Then you work on the snapper and holder getting their jobs down so the kicker can perform. If they are off, kicker automatically has issues that are not his fault. These things are a starting place.