Could it be my sons child?

Patient: My son returned from Afghanistan on Feb 7th 2012. The is the first day he would have been able to have intercourse with his girlfriend. On March 5 2012 she received a positive result for a pregnancy test from a family planning clinic. She gave a LMS date of 2-1-12, they gave her a due date of 11-8-12 and said she is 4.4 wks pregnant (on that day). I have been doing lots of research online and it varies between websites conflicting information of about a week. What I want to know is what is the probability that it is my sons baby, or not? Thank you!!