I don't think it matters how many times you tell a kid not to do something, they will still do it. Kid throws a tantrum, parents punish kid, and kid throws a tantrum a week later.
The way a child reacts to a command depends on how he was raised to handle them. If you made it clear that he was not allowed to expose himself and explained to him the reasons why, I bet he wouldn't do it.
I think if you caught his food for him until he was old enough to control himself, he would be okay. Even a kid can understand that sucking peoples' blood is not normal and that showing himself for what he is would be a bad idea.