Any sexual contact is having some risk but performing oral sex to a guy is a very low but real risk act depending on the circumstances. You don't need to worry unless he had lots of precum coming out at the time of the act and you had mouth problems, like severe gingivitis or mouth wound in general. You need to have sperm on straight contact with a wound in that occasion to get a transmission. There is a very low but real risk. Mouth hygiene is the key on these occasions. And if you re willing to do that again here is an advise. Dont brush your teeth or floss at least two hours before.
You're more than welcome and we all wish you the best.
Oral sex is still considered to be NO RISK for HIV and does NOT require testing.
Oral sex remains a risk for STD's. I suggest you consult with the folks over on the STD Forum.