Thank you very much. From 26 June I only slept with my current boyfriend.
If you didn't sleep with anyone else around August 7 besides your current boyfriend, then he is the father. Ask his mom for some baby pictures of him, and compare them to baby pictures of your child. (Compare photo to photo, not photo to baby in person.)
Sorry, the ultrasound on September 11 (2013 years) showed 6 weeks and 3 days.
I was with my ex on June 19 (2013 years).
I started dating with my current boyfriend on June 26 (2013 years).
I assume "2014.04.30" means April 30, 2014 is your son's birthday, and "2014.06.26" means June 26, 2013, not 2014? (You said you were dating your current boyfriend "from" then, so you can't mean June 26, 2014 which hasn't happened yet?) And then, that your ultrasound was on September 11, 2013? not November 9, 2013?
If your ultrasound was on September 11, 2013 and it gave you a GA of 6 weeks 3 days, your conception would have been on or around August 11, 2013. This would be nowhere close to June 19, when you had sex with your ex. It also would have given you a due date of April 5, which of course doesn't add up unless your son was quite overdue. Can you check your ultrasound date again?
For a baby born at full term but not overdue, here is what the conception calculator says for a full term birth on April 30:
Probable date of ovulation: August 7
Possible dates of conception: August 3 to August 11
Due date: April 30 (40 weeks)