Around 70% of first serves with placement is the goal. Most powerful serve.
At least 82% of second serves. These are usually a bit slower and more spin for control and movement when ball strikes the court.
Consistent improvement in forehand and backhand with goal of equal success and placement whether deep corners or crosscourt.
Equal power and placement with forehand and backhand volleys.
Court recognition, then power and placement of overhead slams.
Finesse and placement with drop shots.
If haven't already. Have him study online videos of service, ground stroke and volley grips.

Beginners and lower level players tend to over practice what they think they do well in order to win and thus weak strokes don't get enough practice. This leads to relying on dominant shots and telegraphing their weaknesses to opponent. Very soon the opponent is focusing on getting ball placement to the weakness and it is game over.

Just a few thoughts. 100% agree that it should be enjoyable work. They can even keep track of their own improvement of shots etc. Then you can just how their session went even if you are present.

Playing with players of almost equal or slightly better ability increases improvement time frame. This does not replace the individual practice or just hitting ground strokes and volleys, overheads etc with a coach or practice partner. Practice games are about improving shots not winning. It is all about improving the # of winning shots in ones repertoire.