The proper time to tell him is 4pm. That way he will already have gone fox hunting, had his afternoon bourbon and read a few verses from his trove of ancient greek poetry: Theocritus, Callimachus, etc.
I kid of course, there are only really three times you can tell your man you’re a virgin. Before sex, during sex or after sex. My advice: before.
It seems you have a sense of shame about not being intercoursed yet and I think you shan’t. Perhaps you’re waiting for the right guy and moment. If that be the case and he be the man, then I suggest bringing up the topic of sex right after a cozy make out session. In that dreamy lull, when you’re wrapped in each other. Tell him you want him to be the one. And let it unfold from there. That’s ideal.
If, however you’re ready to have sex with a dude, just because why not? Then so be it. But then your anxiety is probably coming from other places. You’re worried this dude may be expecting some Tila Tequila pro beast in bed. To that I say hoooey. It’s ok to be new to something, just discuss the topic with a sense of confidence and honesty. Hey dude, I’m down with having sex with you, but I want to make sure we’re on the same page before we’re on the same area rug. For example, where exactly do my feet go? Where will your nut sack be during all this? What’s the deal with the thrusting? Is it for show or something I actually may request?
In other words, open up before you open up. Let your personal decision and ensuing discussion come from a place of truth and actual enthusiasm. And, assuming he’s the right man for you, assume the position; the loss of your V will in turn be replaced by a giant W for the both of you.