It does sound as if you might be pregnant if you are never usually late. At the same time, if you were tense about the contraception issue, that could be a reason.
I would involve him in this because it's happened, it's real life and you hope to have a future together. You haven't tried to trap him so he has nothing to be angry about, really. If one mistake is what he fixates on when you're so anxious, it would say a lot about him.
If he really does want to be a dad, and you are then happy to have a child with him, I think this is the opportunity given your age. Unless you could quite happily live with fertility struggles in a year or two.
The fact that you're not sharing it with him makes me think that it doesn't seem very real to you. I don't think you should make a decision until it does at least feel real.
I don't think the fun part of your relationship is going to be over if you share this but it might be if you don't.
You should feel under no obligation to stay pregnant. If you don't want to have a baby now and he does, I would accept the possibility that the relationship will not survive. I don't think he deserves to know as such but I do think it's too big a secret to keep within an ongoing relationship.
With all that said, I think you'd be taking a risk to have a baby in such a young relationship. Make sure there are no red flags. You would be tied to him forever.