I am so sorry you are having to go through this, but just the fact that you do care and are concerned for Bob says a great deal about you. Please don't give up on him, he really needs you now, even if he technically isn't your cat, he is counting on you to help him.
I know how hard and stressful it is to care for a sick pet, but keeping things as positive as you can will help Bob feel less stressed and be better for him.
I have never heard of Histoplasmosis before, so I Googled it and came up with this info . It does sound like it is treatable, but Bob's kidney condition is surely complicating things, but can be treated as well.
I didn't read how old Bob is, but hopefully he will have the strength to pull through, cats can do amazing things.
I will keep you and Bob in my prayers, please continue to fight this illness with him, he needs someone caring in his corner at this difficult time.
(((HUGS))) to you both. Please keep us updated on his progress.