Welcome to the HIV forum.
Congratulations for a common sense, level headed approach to your sexual behavior. You're doing all the right things. If you told me you had this sort of lapse all the time, I would worry. But remember that even if your partner lied about his HIV status -- which is uncommon -- the risk of transmission during any single episode of unprotected receptive anal sex (with an HIV infected partner) averages around 1 in 200. Now I would not recommend anybody play Russian roulette with a mythical 200-chamber pistol, but most people would survive. Given the relative brevity of the exposure, apparently without intr-rectal ejaculation, your risk was still lower. And it seems very doubtful your partner has HIV.
I'm pretty sure you know all this -- but to cement the message, let's do a back of the envelope calculation. Chance your partner had HIV, despite his strong reassurance: let's say 1%. Chance of transmission, in absence of ejaculation: let's say 1 in 500 (0.2%). Chance of a false negative HIV RNA test at 10 days? I'm not sure, but let's say it's as high as 10%. With these figures, the chance you were infected comes to 0.01 x 0.002 x 0.1 = 0.000002. That's 2 in a million, or 1 in 500,000.
When to test again depends on how much reassurance you would like beyond 1 chance in half a million. At 4 weeks, the standard HIV antibody tests (either rapid or lab-based) detect at least 90% of infections. Would you sleep better knowing your risk were 1 in 5 million instead of 1 in 500,000? If so, get tested at 4 weeks. If you would like to make it 1 in 500 million, have a test at 6-8 weeks, by which time ~99% of infections are detected.
Bottom line: The odds are overwhelmingly in your favor. It seems to me the easiest thing would be for you to have a routine (or rapid) HIV antibody test around 8 weeks after the exposure. In the meantime, you can rest easy and definitely expect a negative result.
Regards-- HHH, MD