I agree that the height isn't such a big deal and I can understand wanting a shorter horse to make it easier for your Dad. You'll want to look for horses that have large barrels and wide chests, heavy bones and feet, and short backs. A horse that's 14 flat and built like a fjord would have an easier time of it than one built like a TB.
If all else failed, you could get him a cart pony and work on finding a suitable mount for him. At least that way, he could still get the interaction with the horses even if he isn't riding.
I know we aren't exactly close together, but if you are ever in my area, let me know. Ya'll are more than welcome to swing by and we have at least 1 bombproof horse that would be more than happy to pack him around for a while