For any one-time event involving unprotected anal sex, the risk is ~0.06% per event. That's for FULL ON intercourse. One second of unprotected contact isn't going to transmit anything - not HIV, nor any STDs. Transmission of HIV generally requires intercourse that includes extensive rubbing of the penis head against the walls of the anus. That didn't happen.
I wouldn't take PEP due to the astronomically unlikely odds that you could get anything, but that's your call. I cannot emphasize enough that you're looking at 1 in MILLIONS of odds. I'm not aware of anyone who got HIV from one second, or even a few seconds, of unprotected penetration.