Have you ever read the James Herriot stories about the Pekingese dog "Tricky-Woo"?
Who was at one stage so spoiled by his lovely elderly lady owner he nearly died? So the vet took him home, let him rest for two days without feeding him - nothing but rest and water. Then let him eat normal active dog rations and run amuck being a dog with the household pack for a while...? He made a big turnaround. Which overjoyed his owner but she went back to her spoiling ways...
Maybe offer him a few days respite, diet and exercise every now and then? It might be the best you can manage.
And find the story - it's a great read!