crossing into Canada and back to US

I *believe* the only requirement is that the dog have a valid rabies certificate. In your shoes what I think I would probably do is go ahead and get the vaccine but take both the old and the new certificates with me, showing no lapse in coverage. I would go ahead and get the vaccine in the unlikely event that something happened to delay my return. But that depends on your comfort level, how long you're returning before his current vaccine expires, etc.

I don't believe a health certificate is required. Not for pets. I believe that requirement is for animals being transported for "commercial" purposes.