Have you done China yet?You might not want to spend a 28 days there,but two weeks possibly,especially if you move around.There is def a lot to see.Hong Kong fits in well with a trip into the mainland.....
A longer trip back home maybe? How much time can he take off in one stretch? How much travel budget do you have (can you fly out to somewhere every month, or should you really limit yourself to 1 or 2 trips that involve airfare). I know my H still has a safari in Africa on his bucket list. With 38 days of vacation, you could easily fit in a nice longer trip there.
Go to the Ice Festival in Harbin in January! (assuming you brought some of your cold weather gear with you)
Other places I'd suggest in China besides the obvious (Beijing, Shanghai, Xian): a water town outside Shanghai, Guilin/Yangshuo, and go to Sichuan and hug a panda like Tofu! I'm so jealous! I always wanted to make the trip, but never had the time when we were there!
And I've forgotten - Did you ever get the chance to take our suggestion to go to Western Australia?