very simple- next time he calls, tell him that you will tell his girlfriend, then hang up.
Here's a thought: Next time he hits on you, ask him if he'd like a blow job. When he pulls his pants down, get on your knees, grab his balls AND YANK THEM TO HIS KNEES. That will put a stop to it. Plus, he'll have to explain to his girlfriend why he's hurting so bad.
Seriously? You want him to fuck you…so bad. And you want us to give you permission. If you didn’t you’d just tell him to piss off and let your friend know what he’s all about. I mean seriously, you claim to not want to crush your gf with this news, but if it was anyone but you he was flirting with you’d tell her in a second. You want his hard dick as much as he wants into your panties. It’s your call, but at least be honest with yourself and don’t insult us with your pretend concern.