Don't judge be but..

I made a mistake, ok not a mistake because I am now having a baby boy, a blessing who I love so much. I was with my ex and we broke up, 4 days later I had a one night stand... no protection was used. My better judgement was not used.. that was the only 2 times I had sex the whole month, so I know I conceived 1 of those 2 times. Ok, so the gestation age of my child at the first ultrasound was closer to the night I slept with my ex.. but 4 days is a very small window to try determine. So needless to say I am unsure as to who fathered my baby!!!! My ex thinks it's his, I have hinted that there may be another possibility. We barely talk anyways, he doesn't really care & neither do I since he is a jerk and he may not be the father anyways. But I am just wondering how to go about telling option # 2? I am 24 weeks pregnant now, I haven't talked to him since that night we slept together because I felt bad about being with 2 guys.. then I find out I am pregnant & I kinda don't know who's he is... I feel guilty so I just don't know how to tell # 2 he may or may not have a son on the way!!!! Should I wait until he gets here & ask him for a dna test or ask my ex to take a dna test? (But I have asked the ex, he is denying a dna test idk why..) I just don't know & the 3d ultrasounds kind of look like # 2. UGHHH I just needed to vent & get a little advice? I love my baby no matter what (: I just don't want to pop up and say hey I'm pregnant & the baby might be yours and then the baby turn out to be my exes..... I hope yall can follow this lol, I know it's a lot of info