What is happening is you're being "slut shamed". It is a form of emotional control that is executed by friends, family, significant others where they bring up the past (usually before they were around) and make you feel bad for it, even though they have no right to do so. Sometimes those doing it don't even realize it.
I wouldn't help him if I were you, I'd let him sort this out on his own, it doesn't matter if you kissed the previous guys for f***ed them, it really isn't his business. Let him battle these demons alone, because the more you help him the more you play into the "slut shame" game that he is playing (he might not know he is doing it, but he is). His only beef he is allowed to have is if you still hang out with these guys or see them (romantically or not), but it doesn't sound like you are. That is all I can say for you, just don't bring it up, don't talk about it, let him deal with it. Believe it or not, it may be your past, but this isn't your problem.
"All is fair in love and war." - Francis Edward Smedley