If you love this man, you have to tell him your age immediately for at least 2 reasons: First, and most important: You are underage. He could be arrested for statutory rape. Second: No lifelong relationship can be built on a lie.
If you won’t tell him immediately, your mother should. Actually, she should have done so and done everything she could to put a stop to your relationship as soon as she found out about it. As difficult as it is to exercise authority when a child like you is bent on doing something unwise, the authorities will see her as responsible for you. Your state’s department of Child Protection Services could get involved.
You may be very mature for your age but this situation is not okay. The mature thing to do is to be honest and to live at home, not with the young man. Focus on your schooling. Get involved with other students your own age.
If your boyfriend loves you, he will understand and will support you in doing age-appropriate things. If the relationship is meant to be, it will still be there when you are 17.
I wish you well.