If you haven't had unprotected intercourse with him since August (or before), you can take a test any time now for your conclusive result.
None of us know, including you, what your ex did or didn't do. If you don't trust him, you were right to break up with him because if you can't trust, it's not time for a relationship with anyone. I'm not sure why talking to his ex is a problem. I'm friends with women I'm no longer with. I still like them as people, that's why I got involved with them in the first place. That doesn't translate to having sex with them. Maybe he has, maybe he hasn't. My point being, yes, get tested for HIV if you've had unprotected sex with someone you don't know the status of. But I would suggest other things are more important and likely, which means emotional issues that sound like they need some work. You don't want to life a whole life like this, counting condoms. Right? Peace.
I meant that you were advised of testing timelines before. There is nothing to do but test if you are concerned.
That is a long epic story with a lot of irrelevant parts, but I think your situation boils down to unprotected vaginal in August, 2020. You can take a duo test after 4 weeks for a conclusive result or any other is conclusive after 12 weeks, so all you need to do is take the duo now or wait until November to take any test. Oral is not a risk for hiv so if that is all you did you had no risk anyway.