Gum disease treatment for a siamese cat

My 12 year old has had chronic gum disease for over three years. He had all but his canine teeth pulled.
This proceedure did not work. He has been on many different antibiotics constantly during this time.
We take him to the Vet every other month. The Vet gives him a shot of cortizone and more antibiotics.
None of this course of treatment has worked. His
weight fluxuates, he drools, stops eating sometimes.
He has had test done for the normal cat problems.
If any one has any suggestions or treatment options
I would appreciate them.

Thank you