Young, you have EVERY right to know basic details of the broken engagement 1) who broke it off (was there a shtar mechilah) 2) how soon after beginning dating did they become engaged and how long did the engagement last, 3)was there interference by his or her parents, and 4)what was the reason given for the breakup, if any?
You may find some answers easier to come by than others, but you are fully entitled to know the basics, and then if you do go forward, he should talk frankly to you about it, and not sidestep the issue, or it will always be an elephant in the room. You are not looking for loshon hara, but you are looking for information that is relevant to your future.
It could be something as simple as they rushed into something they were not ready for. It could be something more complex, like one or the other of them is controlling, or has a serious problem that was not nade known to the other party until after they became engaged. If it was due to parental influence and intererence, that,too, is extremely relevant.
The fact of a broken engagement is not a reason to refrain from going forward. I know a perfectly wonderful young man (who is now happily married and has a child) whose first engagement was broken off by the girl very shortly after it was announced. He is wonderful, and she is a nice girl, too. They just were not right for each other. Much of this is discovered during the engagement period, which really IMO ought to be the COURTSHIP period, with more time alotted to it before getting engaged.
In any case, hatzlacha rabba with this, and if this guy is right for you, all should go forward in the way that is best meant to be for your future happiness.