Dear Laura,
I agree with all the recommendations. Cath's point of breaking the journey with an overnight stay on the east coast before going across the Atlantic is a very good idea.
My mom's main concern when we came to California from Texas was her legs being swollen and, since pressurized cabins cause some people to get swollen, I was a bit concerned, too. Fortunately, a coast to coast trip is usually broken in two legs each way, with each leg lasting about 3 or 4 hours. This provided a good break for her legs to rest.
Of course, the overseas trip will last longer than that. So, make sure you get seats with plenty of room so he can raise his feet. And, although nowadays they don't let people stay too long in the isle even during long trips, try to walk as much as possible during the flight.
Enjoy the trip!
Maria