Most Helpful Guy
You can't. That's guy code for: "If you're the kind of girl that wants to have sex with a guy, 'but only if you're in a relationship,' then I'm telling you straight-up that I'm not the guy for you, move on elsewhere. If you're the kind of girl that doesn't need to be in a relationship to have sex, then let's go for some dinner on Friday, and maybe we can have some dessert back at my place."
Most Helpful Girl
The only way he could change his mind is of he gets to know a girl that he is really into.