disagreed. I think the emphasis should be on the speed of the exercise rather than what muscles are involved. Heavy deadlifts will have a negligible effect on his vertical jump - my hypothesis. However, reverse depth jumps (landing backwards requires more from the hip), top speed sprinting or longer sprints, single legged jumping, will help more.
I actually think that's a good idea.
He might even eliminate all the full squats from his workout and focus on the half squat and that coupling phase. Maybe do some complex training - explosive half squat + jump squat, depth jump + SVJ.
And the backwards depth drops/jumps are also a good idea. To me, when I did stepups I had the craziest calf soreness ever. Why? Because I had to put my leg back backwards on the ground to do my next rep in the stepups. And just that alone gave me huge calf soreness that I never ever get from the calf raises. So imagine what the backwards depth jumps can do for the calves.
Another thing that we omit is the fact that the running 2-leg jump is a different event than a depth jump. It has a lower center of gravity, the plant is more forward/at a different angle etc. There's a different type of loading that occurs, at least from a specificity standpoint.
So this is especially (maybe the MOST) important for him (and possibly for myself as well):
LBSS needs to lower his COM/hips during the penultimate step of the DLRVJ much more than he is now to reduce braking forces in the knee, and well, to jump higher. Learning to DLRVJ properly is the most important thing for him right now, more important than depth jumps, pogos, or any other plyo.
Favoring explosive exercises in the weight room such as jump squats may help a little with improving DLRVJ form, or even just 1/2 squats. They will help with "expressing" strength and improving RFD. I wouldn't recommend box squats however, the amortization needs to be trained with weights.