Might be in trouble!

So my boyfriend and I have been together just over 5 years. We have had a rough relationship break up get back together then break up again. Well this last time we broke up I decided I would try to move on. A week later I started messing around work a co-worker. Well the last time the co-worker and I had sex was January first we did the whole pull-out method. My ex and I decided to try to work it out so I went home on the 5th of January we had unprotected sex. Well now my period is 8 days late. My last period was December 25th it lasted 2 or 3 days. My boyfriend is convinced if I am pregnant he isn't the father because he thinks he can't have kids. His ex wife and him tried for a baby with no luck then she had some fertility treatment done with still no luck. She ended up cheating on him and got pregnant very soon after by the other guy. My bf and I have been trying over a year with no luck. So he honesty thinks there's no way. What are y'alls opinion? I have wanted to have a baby with my bf so bad but idk which one the baby would belong too.