You haven't invited him to meet your family yet, but he certainly is not the guy your mother warned you about. He is someone special in your life, and you're already starting to think of him as a boyfriend. It's time to tell him what he means to you and ask him if he wants to explore being more than your trusty sidekick.
Steps:
Think about whether you are willing to stay together even if he only wants to keep your involvement casual.
Consider the possibility that he is looking for something even more serious than what you have in mind.
Have some idea of what you're looking for before you ask, so that you can stay on task.
Find a special time and place for this important question. Make a date to go somewhere private and comfortable for both of you.
Steps:Tell him that you feel really good about the time that you spend together, and that you find yourself growing closer to him.
Find out how he feels about your dating relationship and what direction he sees it taking.
Share your interest in moving from going out together to something more long-term.
Ask him to be your boyfriend, and explain what that means to you and how you believe that would strengthen you as a couple.
Discuss what kind of commitment each of you is ready to make to the other. How will your dating relationship change? Are you agreeing to see each other exclusively, if you aren't already? Will you live together? Do you simply want to acknowledge that you have reached the next stage of the relationship?
Steps:
Relax in the arms of your new boyfriend, if he accepts.
Give him some time to think about what he wants now if he cannot commit.
Reconsider your needs, and whether they can still be met, if he says that things have been going so well that he doesn't want to risk changing anything.
Be open and honest about your excitement, disappointment or concern related to his response.
Allow for the possibility that either of you might change your mind.
Tips:
Timing is everything. Ask your beau to be your boyfriend when it feels right, not when you reach your 21st date or third month of spending time together. Remember that if you care enough about him to seek more commitment, then you might want to continue enjoying what you have now and see how it grows over time.
Warnings:
Coercion, ultimatums or other forms of pressure may get you what you want in the short run, but they typically do not lead to long-lasting relationships.
source: http://www.strat.ro/HowtoAskSomeonetobeYourBoyfriend.htm