forget about the fact that hes cheating on a girl who he is living with.
The main thing here is to find out if you are pregnant so that you can then make decisions. Simple, go to a chemist, buy a test, follow the instructions and see what the result is.
Oh and if this girlfriend of his is so nasty, why did he move in with her after he slept with you (if im reading you comment about them living together now right)? Surely he would have left her before it go to the co-habitting stage?