Okay, just read this and I just want to be sure I am understanding your situation. You are 18... You live with your boyfriend at his mother's house. You can barely afford to live on the money you have. And he wants you to get pregnant?
I may be completely wrong, and you have probably posted a lot of things I haven't seen, but it sounds like this guy is just afraid of losing you and he wants you to have a baby because then you will be trapped with him.
I think you are absolutely right not to want to have a baby now. It would be a recipe for disaster! You need to get out and be independent for a while, as you say, get a good job, have some fun while you are young, and think about what you want out of life.