Partner can't cum inside me?

I've been having sex with this guy for months now and he has an ejaculation problem, every time we have sex it last for hours. He said he's only ever been able to cum without helping himself twice and both times were with me. But he's never been able to cum inside somebody it always ends up having to end with oral or him helping himself. He gets really frustrated about it. Why does this happen and how can I help it?