Just because a guy says he can't have kids, doesn't mean he can't have kids. I've known plenty of girls who have ended up pregnant that way. Guys tend to make those comments so they can get away without wearing a condom. How old is he? And why on earth would he have had fertility testing done? (insurances don't usually pay for fertility testing and unless you aren't getting someone pregnant, there is no reason to find out if you aren't producing sperm. Ask him what his motility rate was?)
after the baby is born, you'll need to go to social services/child support in your area and tell them the people you slept with and how to get in touch with them. They'll send them notification that they need to come have a paternity test and they will take blood from your baby to test who the father is. Tell them everyone you have slept with, even if that person said they weren't able to have kids, okay?
The formula is usually about 2 weeks after your last period - but that all depends on 'normal' periods that come every 28 days and it sounds like yours weren't consistent or you'd have remembered when it was. If you had sex with more than one guy within a few days apart, it could realistically be either of them. And since you are unsure on any of the dates (other than the 4th of april), only a blood test could tell you.
Good luck.