My son has this as well, but appears to be outgrowing it. It's been such a struggle. He started by ripping the sheet of his crib mattress, tore the outer plastic cover off the mattress and was eating the foam from inside. Double sheets did no good, so we finally ended up having a super large "pillow case" made to fit over the mattress, that velcro'd shut so he couldn't get to it. Then the wall of our utility room was soaked from water damage, and he picked and picked at the wall and made a HUGE hole in it, and would grab handfuls of drywall dust and eat it. He's swallowed bits of plastic and foil wrappers, sharp plastic pieces he bit off of toys, his brother's legos, small rocks, other things. His poop was always colorful.
It's tough even trying to explain this to people, b/c they automatically assume you are not paying attention to them or "letting" them do this. I kept my son buckled into his high chair a lot more often than I cared to, just trying to keep him from eating things that could hurt him. I followed him around with food, when I saw him trying to eat other things I'd stop him and offer the food instead. He still has his moments, but not nearly as intense as it used to be. Hopefully your son will outgrow this too.