Spike helps him relieve the lust which helps for a while, and takes him to Angel to see if a cure can be found (or he knows there's a cure and Angel can help, don't remember the details there). I'm pretty sure the sex expands to include not only Angel but Doyle as well.
Can someone help me out with a link? I can't remember the title or author, and I've scanned through all my BtVS bookmarks on delicious and I don't have it saved.