It sounds more likely that the second guy is the father than the first guy, but your earliest ultrasound suggests you conceived on or around May 12 +/- 5 days, so that does not rule out the first guy entirely. In your shoes I would go through the pregnancy assuming guy 2 is the father, but would definitely get a paternity test with both guys once the baby is born.
I believe it was the 2nd guy who might be the father
Have you had any doctor appointments and ultrasounds?