instead of putting the nori *on* him, next time you see him on the sand bed, put the nori on the sand, gently lift the urchin and put him right on top of it. he'll probably go to town on it. when my tank was new, i supplemented whatever the urchins got off of the glass/rock with nori. you can also rubber band a piece of it to a small rock and drop it in the tank. i would just about bet the farm, the urchin will find it quickly.