so the latest update:
we spoke with an agent on the phone at HK airlines who said it was fine. i kept bringing up the website language on the a350 and infants in business class, and he said because we are not doing a lap seat, but because he has his own seat it is ok. WOW am I glad I paid that extra $1 for his own seat. I had called Expedia repeatedly for clarification, who kept emailing to say they were having trouble getting in touch with HK Airlines. This AM I got this email from Expedia:We have received your request to change the cabin for your son into a business class.

We would like to inform you that upon checking here the airline already changed the flight reservation for your son to a business class of service. Please see details of your flight below.
So we are going for it. We also have screenshots via the HK airlines app that show my son ticketed in business.

Worst case my wife said she will offer to switch seats with someone in coach... I will let you know what happens. Feel free to PM me for a little more info if you need it.