His back would freeze up when he did squats and lunges and since doing legs press and hack squats with his back straight on the pad he has not had any problems with his back, maybe you dont know as much as you think you do.
Why would his back freeze up? Rather than just working around it you should be trying to fix it whilst also giving him a proper workout.
Lets say it is the usual, glutes not firing / tight hip flexors / tight rec fem - do you think limited ROM leg presses (have to be very limited in a guy with a bad back who se hips stay tight down on the leg press pad) are a better long term option than a simple front foot elevated split squat?
For a regular PT client who wants fat loss, strength and mobility more than he really cares about overloading his quads for that extra half inch of leg muscle.
Just my two cents worth meant with no agenda or malice.