So, accidental penetration always seems improbably to me. Normally someone has to push in and it takes effort initially. You sure he really penetrated? That was your only risk in the situation and it is low. A single exposure is less than 2 percent. I wouldn't think you need PEP for this. Was this guy a sex worker? Agree that you should test at 28 days post event with a 4th generation DUO test or any time after that.
There are no doctors here. This is a moderated HIV prevention forum.
Unprotected penetration warrants testing. I understand, it was absolutely brief but it did put you at some amount of risk as anal penetration can certainly rupture rectum's lining, lack of lubrication and thinner tissues increase the risk of friction-related tears in the anus and rectum. Pre-seminal fluid is known to contain some amount of virus and this contact with the lining of rectum can cause risk of transmission, if the partner is originally infected.
PEP is usually recommended for instances where exposure has been with a known positive partner. I think you should consult with a doctor where you live to understand if you need PEP. Since we don't know where you live and what is the HIV demographics in your region not only we can't recommend PEP but also, it is not correct for us to do it from an internet forum. PEP is a matter that essentially needs to be discussed with a doctor and apparently prescribed by them.
I would strongly recommend you to reach out to your partner and speak to him about his HIV sero-status, most people would not lie about it when asked directly. Perhaps, getting him tested won't be a bad idea, if you are sure that he is not in a window period from any exposure that he might have had recently.
It is also understood that sometimes is difficult to reach out to people for above information or there is an element of doubt with the information provided by them, the best case scenario is to take a test at 28 days from the date of exposure, if your doctor does recommend PEP, the soonest a test can be taken is at 28 days from the last dose of PEP, a IV Generation HIV test is recommended for the above stated 28 days window period in both cases.
We wish you the best.