No, not April. More like mid May.
Hm. Because you say "assuming" the May guy is the dad instead of acknowledging that he flat out must be the dad, evidently you wish he wasn't. But there doesn't seem to be any question (unless you had sex with both guys in May). The most obvious reason is that if the dad was the April guy, you would have had your baby by now. (Conception on April 30, the last possible day in April, has a due date of January 21.) Also, all along, your doctors would have been giving you January due dates at every ultrasound. And, you wouldn't have had a period in May. And, yes, sperm would not have lasted three weeks. (But the other reasons are so clear that you don't need to go that far in the analysis.)
Here's hoping the situation turns out to be much better in the long run than it seems to you right now. Sometimes guys will surprise you by being good dads. Maybe Mr. May will turn out fine as a dad even if he flunked as a boyfriend.
Good luck, and congratulations on your upcoming little one.