HZ3006,
No-one can tell you what decision to make about your boyfriend. That is part of the beauty and difficulty of being an adult.
He is probably saying this so that you both go to college open to the possibility of meeting someone who may be a better fit for you. You say that you want a long-term relationship and he may sense that and realize that your partnership is not suited to a long-term pairing.
I know it is hard, but could you enjoy his companionship and friendship now and part on good terms in the fall? As he matures, your friendship may change again and become more what you are looking for in your life. It will also leave you available to meet new people at college if that is your desire.
Most people have a lot of decisions to make their last year in high school. Do you have someone that you can talk to about things?