Sensitive!

My guy really loves going down on me. If he had it his way, he would stay down there for hours. While I love that he loves it so much, I can only handle it for awhile before it starts becoming unenjoyable for me. Its becomes so sensitive and I can't handle it. Does anyone else have this problem? I've even tried to explain it to him but he just doesn't seem to understand. Is there anything I can do, that we can do?