I would suggest, if you are going to get a prenatal test, that you only do one through the DDC or Ravgen, and that you test with both possible fathers. If you only test with one guy and get a negative, will you be convinced it is correct or will you (and he) wonder if it is just an inaccurate answer? Getting a positive for one man and a negative for the other confirms both tests.
Since you had sex with guy 2 on May 6, yes, you absolutely need to test. You would have ovulated on a day that you had living sperm in your body from both men. (I get that as May 6 not May 5 from the dates you've been mentioning: did your ultrasound say an exact number of weeks and days? If so, what was the calendar date of that ultrasound, and what was the number of weeks and days exactly?)
Yes borrice, use either Ravgen or ddc. I used Ravgen and it was correct in my case. Test both potential fathers, Ravgen can do discreet testing as well, you can mail a toothbrush if you wish to test your partner without telling him. Please do not use any other labs as they all have horrible reviews and you do not want to be stressed out about the results for the rest of your pregnancy.