Nice.
Racket head up when he's in the ready position. I'd also slow it down a bit, the interval between shots and allow him to recovery without feeling rushed. Quality over quantity. Do a set of 8 balls, then a 10sec break, 8 balls, 10 second break and so on. If he's wanting to do them longer thats good, means you have his attention and he's keen but don't do as he says.
So in the Wawrinka drill for example that I posted you go through the sequence twice. One sequence is 4 balls (deep forehand, short backhand, deep backhand, short forehand), so 2x then a short break, and repeat. Focus on quality of movement and proper execution of shots. You can do this 5 sets for example in a row, then let him have a short rest. He'll get worn out as it'll become a battle of attrition to keep the quality up. As a coach, once you see his movement and technique start to get sloppy, get on him and remind him what he should be doing right in a quick, short sentence during the drill. "Let's go, keep those feet moving" or "Better recovery!" or "Keep focusing on that backswing" as basic examples. It'll remind him not to lose focus and execute properly what you two are working on.
So basically you can take that one drill, the one Wawrinka is doing in the video which is a RPT Europe/Academia Sanchez-Casal drill, and one day work on technique while doing it, another time work on his recovery so his body is facing you as he moves back to the centre, next time you can work on being more aggressive on the 2 shorter balls and so on, the possibilities are endless really all from that one drill. I don't mean each set to change the intention, but you do like 20min of focusing on one thing in the X drill, then you can completely change drills and do lets say volleys for 15min, and then come back to that initial drill and focus on something else. That's what I meant by intention and not just going through the drill, through the motion.
Hope that helps.
