@modestAvocado4027
You're 18, and while age isn't a number and it's the mind that counts, sure. In this situation, you have a number of life experiences and changes that are ahead of you that will potentially be complicated and even limiting with a boyfriend in tow. It isn't that you care for him less or love him less, or cherish what you've both experienced together but your time to shine is now. With a strong commitment to a boyfriend, perhaps that isn't now as you've stated clearly. There are also value differences, which are core critical requirements that are considered deal breakers for most relationships. In a strong and committed relationship, you'd want shared values -- and the desire to have children is one of those deal-breakers that will end up becoming a thorn between you which you've already called out.
I don't think that this post is really about the reasons why you must do this, but perhaps gaining the courage to terminate your relationship in a way that doesn't harm the underlying friendship and that is going to be tricky as you already know.
There are a lot of variables, unknowns, including how he's going to react, how your life journey will bring you further apart than you could even anticipate now, and of course more as life hands you new hands to play.
If you're going to be the strong and assertive one here, then you'd probably do this via Skype or Video Chat versus text. Do not do text or instant message, no, because that is just a horrible way for someone to receive this type of news, especially if you want to salvage what you both share to some degree.
With Video Chat or Skype, you'll have more flexibility to show emotion, gain instant feedback, and while it's not as perfect as in-person, it isn't as impersonal as other methods. He can at least see in your face that while you do not want to do this, you're firm in your decision and any of his rebuttals to change your mind are not going to work. He will try to change your mind, so be prepared for those rebuttals.
Do not waiver on your task, and that is because if you give any sense of position where you're not sure, or questioning your choice, then it's prone to attack from his angle to try to keep things going the way they are. Therefore, your approach must be direct, firm, with factual reasoning (less emotional reasoning), and with a bit of empathy sprinkled in because this isn't going to be the news that he's going to want to hear.
Perhaps opening the door at that time about potential friendship after this is concluded isn't the right time, because he's already going to be weighing the gravity of the decision that is coming his way. So, I'd hold off on talking about the future unless you know him well enough to know that he could handle hearing about what you'd like in place of your current relationship. However, just be prepared for the potential fact that a relationship of any type after you bring closure to you as a couple is over, may not be possible and it may even take time if it is to happen later down the road.
You should keep such a conversation positive, friendly, completely honest, with respect, and firm. A decision has been made, and it's not negotiable in any terms that keep you both together as a couple, but talking about maintaining friendship is ok, but just be careful about discussing future plans with one another because you do not want to give false hope or any inkling that there may be a possibility of getting back together in the future.
It's a hard talk to have, but at some point, very necessary based on your life's new direction. You're likely making the right call from the sounds of things, and it appears that you've realized that while what you've had has been great -- it is a relationship that has to end because there are other opportunities that may come your way, including someone else or not, that may be what you'd like to explore and this man that you've been with deserves respect and honesty. You do not bring up any other love interests in any aspect of the conversation, even if he asks and that is not about honesty, but not bringing up things that you're not really sure about at the moment. It would be unfair to bring up another guy as a reason because that isn't set yet, there is no direction there and it's only a small feeling, which shouldn't be explored.
You'll have to consider how long term relationships fit into your life's plan for the next few years until you're actually ready for a serious commitment. Usually, guys are not ready for a serious commitment, at least in the US, according to studies until their 30's. So, what I'm suggesting is that long-term committed relationships can certainly wait a while until you're 25-30, there is no need to rush or be tied down at the beginning of your adulthood.
Best of luck!