J.R.
I don't really mind if my son picks his nose in front of me. Having boogers is uncomfortable! But I tell him that it's something that he should never do in public, because people don't like to see that sort of thing.
But eating them? UGH. I guess I would just emphasize that it's not polite and that maybe that can be something he does in private, like going to the bathroom. At least that way you won't know if/when he is doing it. Ignorance is bliss. :)