Said to hear & I friend has been through simalar with her Westie.
I hear of alot about Westies with skin problems & slipping joints these days which is sad as they where once a good hardy wash & wear type breed with mim health problems.
The lifespan seems very shorten also with many not making 10yrs when they should be around well into their teens.
I really have no advise for you other than you will know when enough is enough & no one else can really advise you on that especaily without knowing the dog or situation personally.
Good luck & I am terribly sorry to hear this dogs whole life has been such a struggle due to bad breeding resulting in poor health. Not your fault either & I am sure your've done your upmost to keep him comfortable & happy.
Dogs make everyday life enjoyable...........