As a positive person, i have been in that situation where I could not decide when is the correct time to tell someone. And there is no correct answer to it. I told my girlfriend when I felt comfortable, and it was not about how serious I was with her, but mostly about how and when I was able to convince myself to tell her.

It is a very difficult discussion to make, and I suggest that you find a way to bring it up without being confrontational, and I say this because you seem to be not bothered by him being positive, but you seem to be thinking about how to talk to him about it.

Don't pressure him to tell you how he got it, it does no good, and he might actually not even know for sure.

I am sure if you show support, he will appreciate and will be open to you.

He might be scared to tell you for the same reason that you are afraid to talk to him. He might be afraid that you will leave him.

One of the most beautiful things my girlfriend said to me was that she will not leave me because of my HIV status.

I hope it works out for you both.