Did your period START or END on March 8?
Your doctor is mistaken IF your last period started March 8. Normally you will ovulate between days 11 and 16 of your cycle. If you started your period on the 8th then you likely conceived between the 19th and the 24th. Sperm can survive about 5 days.
The timing works for both men, it could be either. You're going to need a DNA test to find out.
Doctors base dates on your LMP. A due date mid-late December would be correct.
Good luck. Hope it helps.