I'd say his 'maternal grandparents'. It's not exactly wrong to have two 'S but it's poor style unless it's quite unavoidable. That seems unlikely to me: usually we'd use an 'of' instead of the 'S. You could easily write the 'home of his mother's parents'.
There is no grammatical limit to the number of nouns the genitive 's' can be added to. In practice, the limit is about two. In your case, we would say "which is where his maternal grandparents' home is" but we would say, "John's dog's collar broke."
To avoid overuse of the genitive 's' "John's dog's collar's buckle broke." we would use "of" "The buckle of John's dog's collar broke."
(Crosspost with HG)