Advice or something

I apologize in advance for the long post...OK so I have a son who is 16 months me and his daddy have been together since right after I found out I was pregnant and its been great love him to death.....well I have an ex who swears up and down he is my child's father has bugged me from the time my son was a few days about it....there is a VERY SLIGHT chance he could be my ex's the last time we had sex was Jan 4 2014 I started my period the next day Jan 5....well my ovulation time was right around jan 18-23 which me and my child's father had sex on Jan 23 well come Feb 6 I found out I was pregnant and I was 4 weeks everything from day one has added up to be my SO baby well this week we are going for a DNA test on my son with my ex and I'm just terrified with that slight chance that it's going to come out he is my ex's....even though I really don't think so I just have the what ifs in my head....anyone gone through something like this before? I'd never go back to my ex he is mentally abusive and I want nothing to do with him.....just looking for some advice on how to Handle all of this....and I was very very regular when I got pregnant