going by your lmp your 9 2/7weeks pregnant and conception would have occured around Wednesday, May 7, 2008 so to me it sounds like its your exs.
Pregnancy is dated by adding two weeks to the date of possible conception (which typically equates to dating from date of last monthly period in most cases assuming a women ovulates on day 14 of her cycle).
If you conceived on May 6th, you would have been 7 weeks, 2 days pregnant on June 12th. i.e. you would NOT be 5 weeks pregnant because you need to add 2 weeks onto the date of possible conception.
That being said, ultrasounds can be up to 1 week off. In any case, it could be either mans child and the only way to find out who the father is is with a DNA test.
According to this calculator i found:
Results: First Day of Last Menstrual Period:April 23, 2008
Probable Date of Ovulation: May 7, 2008
Possible Dates of Conception: May 3 to May 11, 2008
Due Date: January 28, 2009 (40 weeks)
Hope this helps a little. Good luck!