Visa for visiting family in China

Hello, i'm a HK citizen lives in Vienna, my husband is Australian. We intend to visit my parents who lives in ShenZhen when we fly back to HK this December. Does my husband needs invitation letter? Can the invitation letter be personal one that my parents write? Our son is 7 month old, i assume he needs to appy as well? can one invitation letter invite two person? Appreciate if you let me know the list of documents that is required. Many thanks!