Why did you break up in the first place? Is the reason why you broke up still relevant? For example - if you broke up with him because he was unfaithful - has that changed? If he has not changed, what would be the reason why you would want to be with him now?
If things have changed, do you feel comfortable with the changes? If you're not sure, then go on a semi-date with him, such as movies or dinner. Don't be intimate, if you were before - take sex off the table until you know where you stand. If the date went well and you feel comfortable in continuing, then go for it. Trust your gut feelings on this. If it doesn't feel right, don't continue just because you miss him.