Is it possible to be the reason for a guy to change?

So I've been dating this guy for 8 months now, things have been pretty darn great so far, but he recently told me that he was proud of himself and I asked him why and he said "because he had actually been loyal in our relationship" like what was that supposed to mean?.. So I just went on and then a couple days later the topic abou his exs came up and we began talking about it, I didn't want to know about it but simply part of me did, he told me that he has practically cheated on all of his ex, I was surprised but I didn't say anything about it, and today it came up again and he told me that he cheated on his last girlfriend with 10 different girls, that hurt me a lot to know, because no girl deserves that, he told me that he would never cheat on me though because he really loves me, and he doesn't want to lose me, I do believe him because he looked pretty serious, he said that I changed him, well I believe he in a way he changed himself with some of my help, do you think it's possible for a guy to change? I told him I trust him, but if he ever did cheat on me, that I would break up with him, and he asked if I would take him back and I said no that I wouldn't, and he then said that if we would be done forever if he ever did and I said yes. I think it's better to be honest with him so that he knows that I'f he decides to screw it up, its what going to happen. I wouldn't take him back if he cheated on me, and no girl shouldtake back a cheater. So is it possible that he has changed or will he go back to his old ways?

Oh and We have been dating for 8 months, were both 16, an I do believe that he hasn't cheated on me.
Is it possible to be the reason for a guy to change?
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