I like to use a 3-4rm on GM, ensures that I really keep my form good. Hell my form ususally is best on the 2nd not first rep. I think as long as he works with what REALLY is his max, and not asumed max, while squatting 315, you can stand to do some max effort GM's.
Other lifts that have helped (me personally):
Low Box Squats
Deadstop Front Squats (I do these a lil wider than normal)
Sumo Deadlifts (From my pl squat stance)
Power Squat (Did sets of 5-10 reps...more of a mental thing, putting 7-8plates on each side helped me get past that 4 plate boundary on the barbell)