I'm trying to feed on this moth priest, but no feed option is coming up. I used Vamp Seduction on him, and pressing E talks to him and there is no feed option.

Do you have any vampire mods installed?

I don't believe so. SkyRE might affect it in one way or another, I'm not positive.

edit: Tried disabling SkyRE and it didn't fix anything.

That's awkward, well skyrim + dawnguard for PC just totally crashed here reinstalling ATM. I also have dawngaurd for my xbox did mutable playthroughs and haven't heard of that problem yet.

Perhaps try reloading and doing the fight again? I do not actually remember my character feeding on him, but if that is the way to make him a thrall must have. I stuffed that bit up I think and ended up trying all sorts of things on him. How about pacify?