Sorry for what you have been through. But I don't think that having sex because you afraid of being dumped is a good reason at all to engage in sex, especially with a boyfriend of only a month. You are not ready, and that's all that he needs to know until he has gained your complete trust. Just a month is not enough time I don't think to trust anyone.
I hope you learn more about him, before you give him your heart, or your trust. If he cares, he will certainly understand not being ready for such a big step, for whatever reason. This is one of those things that are easy to get into, and a lot harder to get out of.
You keep a guy with your mind, and personality, not with your body.