How long have you known him?
Where did you meet him?
When you confronted him with the truth what did he say?
What is his age?
This is a pretty serious lie, its not a little white lie so you must ask yourself "why would he lie about something so important" If he is reluctant about talking about his past then it stands to reason that this relationship was not something he wished to talk about. It may have ended badly or there may have been issues that he wants to not think about. Thus the reason for lying about it.
Keeping the truth buried is seldom a good idea. While you could just try to forget about it.....the reality is this is going to eat away at you and possibly cause resentment in your relationship. His reasons for keeping it secret may be vaild......but unless you can live without knowing then it is better to talk about it and get it out in the open.
Sit down with him, instead of creating a conversation where he is defensive and may further lie be upfront and firm. Instead of asking why he lied.......simply let him know you know about the relationship and was wondering why he did not trust you enough to confine in you. Try not to be confrontational and do not be angry. While you have the right to be upset.......this emotion is not going to get you to the truth either. You can let him know that you are hurt, but try to keep the conversation on track with getting to the root of the problem. In the end getting the problem fixed is far more important then arguing about why he lied.