You had sex with your boyfriend on April 20th or day 11 of your cycle. You'd normally be expected to ovulate between days 11 and 16 of your cycle so the timing was very good.
Your ex claims that you MAY have had sex on April 15th or day 6 of your cycle. That's prett early but it is possible you'd get pregnant.
You say you "messed around" on the 26th. That would have been day 17 of your cycle. Again, late but still possible - the egg is viable for about 48 hours.
Odds do favor it being your boyfriends but there is a chance it isn't. Only a DNA test can confirm.
Hope it helps.