Taking an HIV drug defiantly makes things suspicious. However, he still could have tested negative. Sometimes if a person is exposed (especially with a high chance of transmission), the doctors will put the person on HIV meds because the earlier you start, the better. Once the tests are complete over the course of how ever long, if it's negative then, they can stop the meds.
Circumcision can reduce the chances of transmission, but it *does not eliminate it* and should never be seen as a replacement for condoms. I don't see what you being on top would matter, but maybe there is some research out there that I haven't heard of. The fact that he's taking medicine probably reduces your risk as well, but still doesn't eliminate it.
I hope that everything comes out alright. You probably will be okay, but it's still good you are getting the testing done. I also hope you find a nice, responsible guy who you can trust.