This sort of thing is somewhat common in very young children (I read it relates to them discovering their ability to "create" something). In older kids, it may be a sort of what's called reactive attachment disorder. It seems to be common in adopted children, so if your son is adopted, that might be a factor.
The smearing part sometimes might be a kid testing or trying to set boundaries with others. Obviously, it's not desired behavior, but it is treatable, so would make sense to contact a child psychologist. Don't freak out on your son, but I would tell him that you can't allow poop on the walls, so he will have to clean it up. You might also want to ask him why he likes doing it; what he gets out of it. But I think at his age, a therapeutic intervention makes sense.