I don't think you tell him you read it through his emails obviously he doesn't think you are a snoopy girl or he wouldn't have told you to check his email for him, unless he wanted you to see that stuff and then you bring it up.
IDK it could be both ways, I think he should bring it up that way you know you won't embarrass him because he wants to discuss it if he brings it up.
I feel it doesn't bother you he is using it as much as not telling you he is using it.
You said you are happy with the sex now, so leave as is and wait to see if he tells you himself.
It's not what you have, it's what you do with what you have that matters.