Husband can't finish?

For the past few weeks he can't get off. He takes care of me then wears himself out before he gets done. We end up saying, oh well try again later. He's gotten to the point that he could several times, but never actually gets there. Is something wrong with him? I'm not very familiar with male anatomy, but to me there's only two reasons for this: either something's wrong with him health wise, or I'm not enough to get him there. Thoughts?