Of course there is a lot of generalization. As he continues to work on his swing, there will be different things he will need to adjust/work on. If you want to start with something really basic and useful, I'd suggest you emphasize getting him in a proper stance, teaching him the basic approach (body position, stepping into, etc), then tell him to, "swing the knob of the bat" or, "take his hands" to the ball. Often the natural inclination is to mentally try to hit the ball with the sweet-spot on the bat, and this can cause problems. If he goes at the ball with his hands or the knob, the bat will follow appropriately if you have him taking a proper stance and swing.
And absolutely tell him to, "see where it was", in other words, don't try to follow the flight of the ball from the bat...rather preach to him it is more important to keep his eye on where it was when it hit his bat. When he swings through he needs to feel his back shoulder go into his chin after every swing. That will help him keep his head still and train him to really focus on the ball at the point of contact. Tell him you care way more about him doing this than you do where the ball went or how hard he hit it. The where and how hard comes later and will more likely come as a result of him keeping his head still and eyes focused on point of contact.
Praise him every time he keeps his head still and, "sees where it was", whether he hits it well or not, or even hits it at all. Don't act like you give a dang about where the ball went after unless it is to back up the fact he did it by, "seeing where it was". Get his technique right and the results will eventually come. Praise technique...not where the ball went or how far it was hit.
Another also mentioned muscle memory. Dead-spot-on. It will take a certain number of swings to really get it locked in...thousands. I'd also add studies have shown the real difference-maker eventually is the ability to pick up from the pitcher's release point where the ball is likely to be when it's time to hit it. Gotta pick it up well before it actually gets to the plate. Again, this only comes from seeing it thousands of times, so though working from a tee has it's uses, particularly when working on stance, techniques, etc, it will be really important to get him as much practice as you can vs live pitching...not pitching machine, but live where he can begin to develop that part.
I coached a lot of high school and club girl's fast-pitch softball. I've seen plenty of young ladies look great in a cage with a pitching machine, then look lost vs pitchers. I'm convinced it's because they spent way too much time vs a machine and not nearly enough reading the ball coming from a pitcher.