This is Dr.Christian. Welcome to JustAnswer. There are several causes for lethargy, weight loss and the other symptoms you've described. Here are some possibilities:
As you can see there are numerous possibilities but with an outdoor cat getting in to fights feline leukemia and feline aids are the most likely possibilities. A quick blood test can be done for these diseases and if that is negative then more bloodwork make be necessary. If you have more questions please let me know, I'll be online later this afternoon and evening.
he wouldn't come in to eat this morning, but I took food, water and milk to him in the garden this afternoon and made sure that he ate and drank. He did get up to eat. but is moving very tentatively, appears very stiff and sore and is clearly not feeling good. His fur is not shiney and is falling out. He looks like he has been fighting as he has small clumps of fur missing. Do you still think that it feline leukemia or feline aids?
Husband is away with the car and I have a baby not easy to get to the vet until tomorrow evening - is this soon enough - I'll keep him inside and make sure he eats and drinks, plus then I can take both cats and ensure that the other is all right?
Both cats are up to date on shots ie. rabies and cat flu etc, whatever the vet recommends, but I cant find the receipt from their last annual visit in December, but won't they have been innoculated against these or do I specifically have to ask?
If they do have one of these diseases - are they curable?