Bad breath can be an indicator for an oral infection or abscess. There might not be any overt signs; but if it's spreading badly it would explain the symptoms. It could also be a sign of a stomach ailment. Even a solo indoor cat can pick up a parasite infection, it's actually not uncommon. Look in your cat's mouth for any signs of dental issues and take a close look at his stool (although that won't necessarily tell you if there's a parasite, depending on the type).
Also; your cat is at an age where they start to develop some of the older cat conditions, like diabetes and hyperthyroidism. most of the "older cat illnesses" are easily treatable, but you should try to get him checked out by a very nevertheless.
I'll second what Savas said...get kitty to the vet.
It may be something as simple as old teeth gone bad or as complex as a vital organ gone haywire.
Out of the 12 cats my wife and I have had over the years (3 current ones are 6, 7 & 8 years old), all but two lived longer than 15 years.
All of the geriatric cats had systemic problems of one sort or another.
Get kitty to the VET!
I agree with Robjor, We have a kitty visiting at the moment who is 15 years old and he has come here straight from Hospital with all his shaved patches and bits of sticky plaster. He had stopped eating so they took him to the vets but all tests have shown up nothing but he has lost so much weight. He does not have any meat on his bones at all and I have been force feeding him for the last three days. He had been at the vets for 7 days and they could not find anything wrong with him but his blood count was down. We find with the older cats and I have one at the moment at 21 go off their food and the main thing is to get them diagnosed asap as they loose condition very quickly and you might have to force feed them whilst they are feeling off because if they are not maintained during this time like the one I am minding at the moment it takes a lot of intensive caring to get them back up again. Last night after 3 days of syringe feeding he ate a plate of fresh roo meat and licked the gravy from a can of tuna and prawns so I was elated to see he is once again interested in food and I can only hope that he has no vital organ damage from being so starved we have a long way to go to get him back to normal weight but it is a start....bestest.