To keep this short and sweet, I'll just answer your three questions.
1) I read a statistic somewhere, but it's lost now. It was something like 80% of people end up cheating on someone at least once (don't quote me on that; it's from memory). As far as guys specifically, I would guess that the percentage is higher than what it is for girls. Unfortunately, I can't give you much of an answer on this. Looking at a guy's past will definitely help, though.
2) A resounding YES. A lot of things can happen in a few months. You don't want to break up with your boyfriend now only to discover in August that you could have stayed together easily. Give it a chance.
3) I hate to say this, but no. No, no, no, and definitely NO. (That doesn't just go for guys, though - remember that.) Sure, people change. Many people change. Most people change a little bit, and a few people change a lot. But disloyalty is such an elementary personality trait that it's almost ingrained for life once someone is unfaithful the first time. I would give him a chance, but don't fall into a trap.
That's about it. Good luck.