Your doctor is assuming May 28 for your LMP as well, based upon the due date.
If so, then it is unlikely Guy A is the father.
You would have had sex with Guy B on day 12 of your cycle. Normally you will ovulate between days 11 and 16 so the timing was very good.
Without definite dates it is impossible to say for sure that Guy B is the father. A DNA test will be needed to confirm paternity.