Why put yourself and your BF through all this stress without knowing for sure?
Go to the drugstore and get a test, take the test, and then talk to him.
It takes two to get pregnant and if neither one thought of contraception, then there is a problem. Did you two talk of plans for the future, or did you just assume that once engaged, and then once pregnant all will be hunky-dorey?
If you 'accidentally' got pregnant and hope all things will go your way, you are being a bit selfish. This includes the future of at least two people, if not three, and such choices sometimes don't pan out the way you want.
Being practically forced to fatherhood is a shock to any man, no matter what age, and if he feels that you gave him no choice in this he has a right to be shocked, disappointed and not 'be there for you'. Who is there for him right now during his time of stress, frustration and worry.
So, once the test is done, sit down and talk about the future, whether you have one together or not depends on your ability to really communicate with each other.
Good luck, and keep us posted.

Jumping the gun and stess is not healthy in relationships.