Well I think you should explain to him is love doesn't equal sex, and sex doesn't equal love. If this girl really likes him she should have respect enough to understand his wishes of staying pure. Pressuring him gives me the impression, shes in it for sex, she has no respect for him, or/and she just needs a good talking to also.
But this boys responsibility is to really communicate with her and explain to her that saying pure is an important goal to him and that her pressure is not helping him at all, because it is obviously not. And from there on out if she continues to pressure him she obviously doesnt care what he wants and he should break it of with her and just tell her, something like, if you can not support my decision of wanting to stay pure in this relationship then its not going to work... and if she later decides she wants him back and that she will actually try then they could take another shot at the relationship.
And he should talk to her about a few things that would be deal breakers, about how he is interested in getting in a long term, committed MARRIAGE and also talk about all of his morals and ideals. Because chances are after you tell a person about that stuff they will either accept it or be turn away from it. You know?
And you guys are like what 14? Something like that? Thats really young to be ask/pressuring someone into sex, definitely a boy, because it seems harder for boys definitely if they are with someone they are attracted too.
Another piece of advice for him, its to avoid situations that give the chance for things to escalate. Like my boyfriend he sometimes confides in me about how he is worried and hopes that we never are stupid enough to get caught in the passion of them moment. But I always tell him, "As long as we are not STUPID, like- "hey aaron no one is at your grandmas house, lets go there, and into the dark bedroom, dont worry nothing will happen....oh wow aaron I can seem to stop myself...-then a dumb face-" no we should be like "you want to go to you grandmas house? no one is there?" and have him or me go 'no because then something could ~happen~."" If you know what I mean? Stay in public places, if you happen to be alone together make sure its not very long and you try to get in a place where it is public. Or if you want to be alone but still in public you can just go to the park or something where there is people (you dont know) but its not like you can actually do anything bad. Me and my boyfriend go to the park all the time. Because we can talk there but its not like isolated.
Is this boy a Christian? How long has he been with this girl? Have they said they loved each other?