It's called "coprophagia" in case you want to look it up on the internet.
This isn't completely uncommon for 2 year olds. In older kid's it's often related to autism or mental health issues.
You also see a lot of it in kids in orphanages, and dogs who are kept in kennels all day long with no stimulation - it's a boredom thing. I don't think it's usually associated with nutritional deficiency.
At this point, your son sounds like one of those little guys who will just outgrow it. In the meantime, can you put him in an outfit he can't get out of? My oldest son would smear poop when he was about 10 months old at naptime, and I created little overalls with an actual paddlock snap closure that uses those teeny little dollar store key lock paddlocks.
This too, shall pass. I was buying greeting cards the other day and there was one that had a pic of two little girls in the kitchen with chocolate smeared on their faces. The front said "They looked so cute Mom just couldn't be mad" and the inside said "Until she realized it was poop, not chocolate".
So anyway, you're not alone. ;D
Wonder if he is really eating it or just smelling it or rubbing around in it? Anyway, perhaps try something like huggies. they are much tighter an harder to get into.