If you were supposed to get your period on the 6th, I doubt you were ovulating on the 2nd when you had sex with this other guy. I mean it depends on how regular your cycle is but odds are it’s your boyfriend’s baby. And if the sonogram said 9 weeks in early June then you conceived way earlier than May, even before you got in the fight.
There’s no harm in getting a DNA test just to be sure but I wouldn’t be too worried about it. Just talk to your ob/gyn and make sure you didn’t get any STDs from that guy- you said he used a condom, but still, check, because they can cause pregnancy complications.