The problem with trying to take the ovulation surge as proof of conception date is that if I remember correctly, the ovulation surge happens before ovulation. It triggers ovulation, it doesn't identify the moment of ovulation. Look on the website from an ovulation test kit company and you will probably see that they advise that after you get evidence of a surge, then you can expect ovulation in, what, 24 to 36 hours? Something like that. The longer that time frame, the closer it puts you to the sex with your ex.
How early in the pregnancy did you get the due date of April 25? Another complicating factor in the story is that if it was later rather than earlier, it can't be relied on to pinpoint when an estimated conception date was.
Long story short, yes, you still do need to do a DNA test.
It does seem more likely that your boyfriend is the father, but it also seems like it's okay if it's either guy - since you knew you were ovulating, and were testing for it, and had sex with your ex more than once in that window.
Anyway, yes, you'll have to do a DNA test to determine which guy it is.