Ask your vets if they have heard of Transfer Factor - it worked wonders for a cat here, and also extended the quality of life for another cat(who was VERY hard to medicate, despite hiding it in any kind of food whatever!).

This is a long thread, but it is the cat Haggis's story (three guesses where he and his fur brother Hagrid live!):

http://petoftheday.com/talk/showthread.php?t=147840

Darling Darby sounds like a fighter - all bodes well! Prayers and DO keep us posted.

um...any pics of this handsome sweet boy? You can just post them as attachments, NP.