I am glad you got the answers, and are 100% sure of the paternity of your baby. Now you can focus on the future. Do not engage with the ex, and ignore any attempts he makes to contact you. That is a lot of stress for you and the little one. I wish you all the best!
The guy does not know much. Ultrasound measurements late in pregnancy can be off. But ultrasound measurements done in early pregnancy would be able to tell the difference between a baby from sex January 19 and a baby from sex February 7 and 12. When you got the gestational age accurate to February, did they tell you an estimated due date? What was it?