With a due date of 27 August your LMP would have been around 20 November. You'd likely have ovulated between 1 December and 5 December.
You DO NOT conceive the same day. It can take SEVERAL days to conceive. Sperm can survive up to about 5 days as well. The egg, only 48 hours or so.
Odds are your EX is the father as his sperm would still be viable when you likely ovulated. Your boyfriends sperm would most likely have died before ovulation.
You need a DNA test to confirm.