I am a guy that's cheated. So let me give you my opinion. So these people here constantly bashing human behavior see something. People cheat for one of two reasons: insecurity and sex drive. You have to ask yourself rationally "why did he do those things?". The answer to that should decide your next move... The danger comes when you have someone very insecure in themselves, typically quite a nice guy syndrome type and someone who talks to lots of different girls (the same type is in girls btw). Sex should't be an issue if there's communication of what you both want.
Let me tell you something everyone constantly sweeps under the carpet. People are not naturally monogamistic, it simply isn't the best way to reproduce. It's a social boundary we've adopted, just like not killing your friend when he uses your iPod... Looking for someone who "wouldn't cheat" is like trying to find someone who doesn't have the capacity to get angry.It just doesn't work like that. People also lie, all the time, and it's rarely malicious.
Should you worry about previous actions? That depends on why they did it. If there seemed to be nothing wrong in the relationship and they did it anyway, then yeah, because there's a high chance they're very insecure in themselves and will seek the same thing if they don't learn to like themselves. If it turns out they had sex once every two weeks, she was constantly over jealous and he didn't love her anymore then... You get me?
IF you think that the causes were genuinely not all him and you go into a relationship you have to accept now for now and let old times go. If you get into the relationship and then snoop, get suspicious, constantly ask about going out and be over jealous and let your mind run away from you. You'll essentially treat him the way he's always been treated, and he's going to behave the way he's always behaved... (behavior science :P) That constant feeling you don't trust them is just translated in someones mind to "they don't think Ii'm good enough because I did x or whatever". This just makes them more insecure, weakens the bond they have with you and makes it more likely they'll seek someone who validates them elsewhere. See the circle?
You need to give it some time, don't go exclusive quickly, don't go gf/bf quickly to make sure he's sexually ready. When it's clear you have all his time and you're ready for a relationship. DONT BRING UP THE PAST. All he'll think is you're judging him in your mind to the standard of the past and you don't really love him. The best way to handle him (and all people) is to trust him and give the power in his hands that he's going to do the right thing.
There is a lot to be said that, it's not the leopard that's never changing. It's the fact he's never been shown kindness from the people that keep him trapped in a cage and whip him. Keeping him the same he's always been...
I am by no means telling you to go blindly. Because if this guy doesn't feel guilt, is emotionally cold or has sociopathic tendencies then no normal behavior thinking won't actually apply. I hope this wasn't too long it's just my thoughts :L
Mehhhhh,