Yes, you definitely need to tell him. You need to just start the conversation and say something like, "i made the mistake of lying to you about my age...". Just let him know you are scared to lose a love like him because you want more than anything for him to be a part of you and your baby's life.
Just be prepared, he is going to be upset at first. A lie is a lie, and you will just have to be honest with yourself and let him know that, because really, who likes to be lied to? Just reverse the situation to where you are in his place, and that will help you to empathise with his feelings. Good luck to you!