Honestly if you want an answer you may not like it

TAKE THE CAT TO A VET  You just described major symptoms of possible several
medical problems

I won't say keep the cat inside for I do not know where you live in the
world.

Take your cat to the vet. Most vets are more than willing to smaller monthly
payments when settling your bill. There's really no reason not to have your
cat checked out. Often, when a cat suddenly changes his litter box habits,
it means he's ill. So take him to a vet.

Every day you need to clean the litter box, removing all used litter and
waste. Use a litter scoop for this. Add some baking soda every other day,
mix it in, to keep odor down. Then dump out the old litter each week. Make
sure your litter box is clean, the box itself must be washed every week with
hot water and antibacterial soap.Never put used litter back into a freshly
washed litter pan, only put new, unused litter.

If your cat isn't neutered then that may be why he keeps getting in fights.
The best way to care for your cat and keep him safe is to keep him indoors.
That's the only way to prevent future fights, accidents, acquiring diseases,
etc. The only way. Since you let your cat outdoors, make sure he has a
safety collar and ID tag so if he gets lost he can be returned to you.

I'd take the cat to the vet.   Maybe the cat got into a fight and
suffered internal damage?

At a minumum the vet should do xrays, lab tests.  

Maybe where the cat gets into fights so often that it is time
to keep the cat indoors?