If he needs help pottying or cleaning up after an accident, it makes no sense to tell him nobody should be looking at his boy bits. What does make sense is that nobody should be touching him in ways that make him feel "bad," worried, or scared, not even his mommy or daddy. Assure him that if that ever happens, he should tell the toucher to stop, and tell you or Daddy about it right away.
Nakedness is not really a big deal for kids at this age, and in fact "booty camps" during which groups of kids are potty trained often has them running nekked in front of each other and the teachers. They notice differences in genitals, and are curious about them, but their ideas are not yet sexualized, and so there's no real shame or awkwardness about it yet.
I think it probably does children, as a whole, a disservice to be too anxious about "all" the perverts in the world. All but the tiniest fraction of adults would much rather help and protect a child than take advantage of his innocence. Kids this age are literal, and need clear black and white guidelines. Too much fear and anxiety about life and its potential dangers are more harmful than helpful.