Your doctor has estimated your last period as starting approximately December 22.
Normally you will ovulate between days 11 and 16 of your cycle, so between Jan 2 and Jan 7. Sperm can survive about 5 days, the egg for 48 hours. Your timing to get pregnant with your boyfriend would have been "very good."
Having sex before Christmas it likely would have been too early.
Only a DNA test can confirm paternity but it appears as though your BF is the father.
Hope it helps.