I would try prone planks, from either the elbows and knees or elbows and toes if he tolerates it. Also, using Romanian Deadlifts (opposite of the Goblet squats using a DB, hanging it low). Quite a few people report decreased symptoms afterward, I suppose because of the firing of the erectors. I've also used reverse GH raise, upper body supported and both LE's raising.
Just my $.02. Good luck!