HIV being passed orally is extremely rare... If you're feeling concerned, you can certainly get tested. And then you should use the Internet to read up from reliable medical sources about how HIV and other STIs are transmitted. Then you'll have learned that oral transmission is extremely, almost vanishingly, rare. You would also learn about PrEP -- Pre-Exposure Prophylaxis -- drugs like Truvada or Descovy that you can take a few days before and after having sex and nearly eliminate any risk of HIV infection.
While HIV is probably not much of a risk for you from that encounter, you are far more likely to contract another STI. Lots of people don't even know they have some STIs and the moment you put your mouth on his dick (whether he cums or not), you could catch it. So you should get tested, not just after sex, but regularly if you're going to be sexually active... about every 3 months, or monthly if you're very active. Most STIs can be treated very quickly and easily... As a baby gay, you really need to learn about this stuff. Luckily the Internet has more detailed and informative resources available than chat boards about big dicks. ;-)