Here's the problem: Back inflation rigs don't go well with typical open water classes. Because typical open water classes are taught by typical Open Water instructors (and those instructor are taught by typical certifying agencies), and they (both the instructors and the agencies) like the students on their knees for skills. (Don't ask why they like their students in this position, they just do.)
A back inflation rig will make the student tend to pitch forward, and the student will have to wave their hands around to keep themselves planted on their knees. And then the instructor will keep telling them to stop waving their hands.
Really, unless you have an instructor that allows for skills to be done in natural diving positions, (and since I am not teaching the class, you probably will not have such an instructor), you are better off letting him use the standard rental gear for the class.
The minute he is out of the class go diving with him in the BP/W, and tell him to never ever put his knees on the bottom again, because it is a useless thing that Open Water instructors force students to do. No one should ever put their knees on the bottom, least of all in a training class. But instructors (and certifying agenices) are set in their ways, and PADI/NAUI/etc still force their would-be instructors to learn and be tested for their instructor rating with students in this position.
