My opinion may not be worth loads, but I'll voice it anyway. Your BF was just as responsible for birth control as you. If he didn't want a child, he should have been more diligent. (Yes, water under the bridge now.) That said, a man who will threaten to walk if you choose to keep this baby....giving you a very manipulative ultimatum....is one I'd let walk. Every relationship has it high points and its low points. Married or not, you've got some mental/emotional version of "for better/for worse", else you wouldn't be together. Hitting a low point like an unexpected pregnancy lets you know if your partner is one that will stand with you when times are rough, or if he's the type to run and leave you standing.
I'd not only let him leave; I'd pack for him.
I have to second MaryRuth says!!! You are so blessed, God doesn't make mistakes and while this baby may be a surprise to you, God never says "well, I sure didn't see that comin'" You and the baby are what's important. The bf has a choice to be a part or get out, you and the baby don't. I have been where you are, only I was 18. I 'forced' the wrong man to marry me and 38 years later I still bear the scars (emotional scars). I know it is scary, exciting but you have the support of your mom...trust me that means LOTS. Hollow is so right that is so manipulative and down right mean of him to place all the burden on you, he doesn't love you or he wouldn't do that and would respect your desires and decisions.
This is an exciting time for you, You need to try to relax and enjoy this pregnancy for your and the baby's health and emotional well being.
Good luck to you and your baby!