This doesn't sound good. I can't say that I fully understand what your vet is seeing. Do they feel that the ear infection
is the result of a systemic infection or the cause of one? To be honest, pretty much most every ear infection that I've seen is its own separate infection. I've never seen one spread elsewhere (not that they can't but when they do it is usually through the blood stream to the kidneys
, heart valves, or liver. Not necessarily would I suspect the oral cavity as a seeding or initial source point.
I'm curious to know what they did to determine that he has a systemic infection. Did they just do a CBC and see a high white blood cell count? The sores in the mouth and on his teeth, could that be just very progressed dental disease. I've seen a lot of cats with very severe dental disease, and in all reality, if the proper dental prophylaxis and care is done, they can do very, very well.
I honestly feel that at this point, your being in a position to make a quality of life decision is one that should get a second opinion. Not that your vet is wrong in their assessment, but that decision is one that you can't come back from if it is made. If your vet feels that he doesn't have a very good chance of making it through the treatments with a good quality of life afterwards, then maybe another vet in your area can give you a different opinion based on different experiences.
10 years of age isn't what I'd call an old cat by any means. Don't feel bad about getting a second opinion. That is something that you have a right too. If you want to give Riddle a chance, then it sounds like this is what you should do.