Have you checked him for fleas? Used any flea powder or solution recently? Have you also checked to make sure he does not have any ticks on him anywhere?
I would not let him back outside, and if you can, I would check him all over, comb him and if you find any fleas or ticks, then buy something from your vet for that problem.
Losing fur, could be because of the seasons changing, but it could also indicate certain diseases he could possibly have, especially since he is a pretty old cat.
Try giving him canned cat food, that is a good brand, mostly chicken or turkey. Also, do you think he could have been bitten by another animal, or a snake, or some other rodent?
Was he tested also for FIV/FeLV???
Sometimes, sudden heart problems, can cause problems with not being able to move, but, if he is doing better, I would still keep him, in, and if you can, as I said, check him for fleas and ticks, and a possible bite from something.
God bless and good luck!