I'm 15, and my brother is 12, so hopefully I'll never have to have this conversation with him, but ya never know.
If my brother was going to have sex at 14, I would try my best to convince him out of it. I think a good age to start having any kind of sexual acts is about 16 or 17. By that time, you should be mature enough to understand the consequences of unprotected sex, and the real reason we have it. But 14? That's way to low.
If he was completely convinced on it, I would tell him to make sure to use protection, and he got her pregnant, I would kick his ass and make him feel the consequences of his actions. I would make sure that he stood and acted like a man and cared for the kid. If I made that mistake, that's exactly what I would want my dad to do to me. I would make sure that his "partner" was also using some form of protection, and that she knew if she got pregnant, I would make sure they stood up to their HUGE mistake, and acted like adults.