Why can't they clean his teeth? I would like to suggest you consult a specialist at a university hospital, such as UPenn. While antibiotics can help prevent the infections, it won't stop plaque and tartar from forming. There are also, of course, side effects to all antibiotics. You said over a dollar a day– but even 2 per pill x 2 a day for 7 says is under $30. In a year, that's less than many here pay for cleaning once a year. You can get it online from places like http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=9637 for 99 cents a capsule and probably cheaper in bulk elsewhere though so you can cut the cost a lot by ordering them.
DAILY brushing (plaque hardens in less than 36 hours making it impossible to just brush off) may help keep most the problems away. It doesn't take a lot… I count to 20 on each section, can be done in less than 3 mins every night. (I have a dog with an autoimmune disorder, gets abscesses/gum inflammation from plaque so I have had to do it for years).
I have to congratulate you on your senior dog. Sounds like both your family and he are lucky to have the other.