How Can The Paternity Of The Fetus Be Determined?

Hello, My doctor have me the due date of Febuary 28, 2015. Then when i went back for another ultra sound they gave me the due date of March 4, 2015. I only had sex with my boyfriend June 10, 11, or 12th. We broke up then i had unprotected sex with someone June 15 . Who is more likely to be the father. Based on the due date march 4 my conception date would be on or aound June 11. Based on the due date march 4 my conception date would be on or around June 7th but i didnt have no sex till June 11 or 12. help me please!