Hi Bonnie, and welcome to the site. You are a very generous mother!
I don't have a lot of familiarity with the exact model you are looking at. Have you considered other makes and models, as getting focused too early in the process of buying a "first boat" could lead to a less than optimal decision? I am not saying anything bad about this boat, just trying to ensure that you end up with a boat that meets the needs and wants of your son.
As far as generators go, I have a 9 kilowatt Onan as well. If you want and regularly use air conditioning, a 9 KW is probably OK. I dont' have AC, so as Dave says, I have to run electric heaters on deck sometimes (on hot summer days) to load it up properly. I would be better with about a 5-6 KW. My boat is a Nordic Tug 37, so about 40 feet overall, so a similar size to what you are looking at. Northern Lights makes a good unit.
Good luck in your hunt, and your son is a lucky guy!