Welcome to the HIV forum. I'll try to help.
I don't mean to alarm you, but your risk could be quite high. It is likely your partner had a new HIV infection -- although of course this depends on how recently he had been tested with negative results. If it was quite recent, there is a good chance he was in the seroconversion window during your exposure, i.e. the period between infection and when the HIV antibody tests turn positive. That is the period when people with HIV have among the highest viral loads and are more infectious than at any other time. And of course unprotected anal sex is the highest risk for HIV prevention of all sexual practices.
However, even with that bad news, the odds are in your favor. The overall efficiency of HIV infection is low, and even with high viral loads, the chance of infection is low for any particular sexual exposure.
So you definitely need HIV testing. An oral fluids antibody test is OK, but not ideal. If negative, it will be highly reassuring. However, the oral test has a relatively high risk of giving false positive results -- and in a relatively high risk situation, you really should avoid the slight chance of getting an initially positive results that actually is negative. (Think what you would go through in the few days it would take to sort it out with additional tests.) That said, the most important thing is to be tested right away, preferably with a blood test for HIV antibody (or perhaps a duo test, which detects HIV p24 antigen in addition to antibody).
In any case, please do not attempt to select your own tests. See an HIV-knowledgeable clinic or provider and follow that advice about which tests are best. Most likely the result will be negative, and negative results at 6 weeks are highly reassuring. Nevertheless, in a situation this risky, a final test should be done around 3 months after exposure to be completely certain you weren't infected.
You probably don't need "the lecture": you know it was dumb to let your judgment slide and change to unprotected sex. Your basic intent for safe sex is excellent, with both condom use for anal sex and discussing HIV status with your partners. Try to avoid even temporary lapses in the future!
Please return and let us know your test result. Good luck-- HHH, MD
This morning (6 weeks post-exposure) I got a rapid blood test from the local health department. The result came back negative. I plan to get tested again in another month or so to be sure, but how "highly reassuring" is the negative result after 6 weeks?
Thanks again for your help.
That's very good news; I'm glad to hear it. Congratulations! The negative result is at least 98% reliable. You should be tested again at about 3 months, but you can be very confident that result also will be negative. Please avoid sex until then, but don't lose any more sleep about this event. Just let it be a lesson about sexual safety in the future.