"Where in Hong Kong is his home?"
I think there's a subtle difference in meaning here. 他的香港家在哪儿 = Where is his Hong Kong home? 他的家在香港哪儿 = Where in Hong Kong is his home? The first asks the location of a thing (Hong Kong home) and the second asks the location of a thing (home) WITHIN an area (Hong Kong). HTH
I believe that "de" has to convey ownership, and this order you present conveys ownership of Hong Kong instead of ownership of his house/home, so "de" needs to be before "jia". I understand the desire to feel like "his Hong Kong house" is suitable, because it is suitable in English, but the order can't be written that way with Chinese grammar rules.