He shouldn’t have made you feel like a leper, absolutely.
That said, some people are just uncomfortable around blood or anything that looks like it. Nausea, fainting, other things that can ruin a good time. It’s an involuntarily reaction: It’s nobody’s fault, but you can’t help feeling freaked out. Also consider that it’s a pretty sensitive area: If a guy seeing blood coming out of his crotch, that’s “Get to the doctor pronto” territory, so there will be, unfortunately, associations.
He does need to be a grown-up about it: These things will happen and it’s a natural process. But don’t expect him to get there all at once.