You need to be in Hong Kong on the date of the application and the date of the appointment. What you do in between is up to you.
And yes, they do check. We left HK for a six-week vacation 2 days after applying and they did review our file to confirm when we left vs. when we applied. I also had to do some appointment juggling to make sure we were back in time for the appointment.
I should also note if there is any problem with the application or need for additional documents that request letter will go to your Hong Kong address on the form. You may need to have someone check the mail.
Isn't there also something on the form that requires you to explain all absences over XX months? His family might have a problem filling that out if they have been gone for a few months from HK without visiting.
Going to be tricky...