He wants to do oral on me...advice?

It'll be a new experience. I'm not sure about the female anatomy with being wet, but I know from experience that girls tend to bleed while doing it in the horizontal position. The mess gets on the sheets. I'd try vert next time if I had to experiment that way again. Try vertical and spread a tarp over the bed. The downside is that one of you has to be upside-down.