No, honey. There would be no point in hurting his feelings with this information. That would be using him just so you can feel better, it is not kind to him. You already did one bad thing (oral sex with someone when you were someone else's fiance), don't make it two bad things (telling your former fiance that you did this). Just let it be one of those big life lessons for you, that you don't want to be a cheater ever again, and don't want to hurt another person.
I'm not sure I understand the motivation to come clean. It sounds like there's some desire to hurt him, the way he hurt you by not being an attentive lover, and by flirting with other women in your presence.
Sadly, this is how friends with benefits usually ends. Although both partners seem to think they won't have possessive feelings, and hurt feelings, one partner usually does. I don't see how telling him you cheated on him while you were wasted would accomplish anything, and it's doubtful he would react out of hurt and jealousy. It seems he would feel even more justified in treating you as cavalierly as he did.
This is as they say, a "teachable moment". Friends with benefits will break our hearts.
Here's the rule about whether to "come clean" with a past injustice you did another person. Will your admission make the other person feel better?
In this case, it's a maybe. He might feel better about the way he treated you, and might feel justified in not respecting your relationship if he knew you also didn't respect your relationship. So it's 50-50. Your behavior would give him permission to feel like you treated him the same way he treated you, and would likely make him feel justified in not pursuing a more affectionate and profound relationship instead of the very shallow and unfulfilling physical one. If he knew you behaved as if there were no strings, he'd feel it was acceptable that he also did that.
So it's your choice.