This 100%. Was the message the last message sent, or did he have to open the thread to see any mention of Billy? If mention of Billy was visible through the preview of the last message sent, and he was using your phone to message someone, then I could see how there could be at least a little bit of doubt that they were going through your phone. But it doesn't seem like that's the case. He read the texts either way, so even if he did happen upon the message thread by accident he still read through it on purpose.

If you really don't think that he would do such a thing, I would send a message to your friend with something along the lines of "X accidentally saw our messages, I pretended Billy was a vibrator and he believed me". Then bury that deep in your messages so he'd surely have to scroll up. I would put money on it that he sees that.