It doesn't matter if he pulls out. The point of condoms are to be used from start to finish. As for when you conceived, we can't say. hCG varies widely from each female. As an idea though, it sounds like you're probably 4-5 weeks pregnant. Means conception happened maybe 2-3 weeks ago. Maybe 4, as sperm can live up to 7 days in a female.
Chances are good if you were ovulating. There are at least 10,000 sperms in precum alone it only takes one to get pregnant. It also sounds like you would have conceived fairly recently if was to go on your levels alone but, the best way to find out is with an ultrasound. I hope everything works out for the best for you.