As SailingDog mentioned, cold is a very important factor for liveaboards. Without much insulation, most boats will condense a lot of moisture in very cold conditions. The winters on the Chesapeake often are rather cold. The typical cabin top has very little insulation with large sections of fiberglass covered plywood in some sections and maybe a small amount of foam under the headliner. Windows are single glazed. Things like this will precipitate like crazy when you heat the interior and just sit around breathing out moisture. Without serious remedial measures, mildew forms and that's not a pretty situation. For the winter the deck needs to be cocooned with at least a well fitting set of customized tarps. A serious dehumidifier will be needed inside.
He may be on to something with the double enders, as they tend to be blue water boats with more insulation even in the topsides (hull above the waterline).
I think your son needs to seek out people who have wintered on a sailboat around here and learn from their experience on how to cope when we have a really nasty winter. I suppose there are some vessels which are much better insulated but I believe you will find that they are quite pricey. If he is going to actually live aboard throughout the Chesapeake winters, I think that the strongest determining factor should be the winter suitability of the vessel at tbe expense of other things such as sailing performance and lower cost.
Another thing that happens is that to sail a liveaboard vessel a bunch of things need to be stowed (yeah, that tiltey thing). Then to convert back to the cozy liveboard situation the same items need to be unstowed. I have even heard of liveaboards who end up with two sailboats: one to live on and a smaller one to sail.
For online resources you can use www.yachtworld.com
. That accesses a worldwide database with listings from over 15,000 brokerages. It also has a power search mechanism if you know what you are looking for.