Denna was a rock-aholic until she was about 5 or 6 months old... we did a lot of "leave it" exercises and redirected her to balls anytime we saw her going for the rocks - our yard had 3/4"-crushed rock all around the edges, so she had LOTS of rocks to go for, too... so it was a constant battle.
You could see if adding some Diatomaceous Earth to his diet helps - it's high in silica, which might be what he's after?
Not saying it will help... but might be worth a shot... You can get Food Grade DE at some yuppie all-natural pet stores, or online. I'd use just 1T per day and see if it makes a difference... might take a few days if it does anything at all.
