I don't think it is a standard that the larger the person, the bigger the horse should be. Instead, it is about the build and weight of the horse. Draft horses are geared for pulling a cart instead of riding, their backs arent necessarily stronger than other horses. You should look for a horse that is not swaybacked, has thick boned legs, and a broader body. Also remember the old adage that a good seat makes the difference. If your husband has a good seat, it will be much more comfortable for the horse than if he is bouncing everywhere. A good seat will distribute weight more evenly acrosss the horses back.
Of course there are percentages that horses should carry on their backs relative to their weights. Obviously, a 14 hand horse with skinny legs shouldnt have that heavy of a rider.
Personally, I am a heavier rider and I ride a Morgan Qtr cross with thick boned legs. He carries me up and down our hills with ease! Don't get discouraged, you will find the right mount for him without having to look for anything special. :o)
You just need a solid horse. Is he tall? If so you'll want a horse with a fairly large barrel. Generally a horse can carry 20% of his weight WHEN HE IS FIT! Just because a horse is 200lbs over weight doesn't mean he can carry that extra 20% lol
You can go for a solid horse of any breed, but often QH, Standardbred, Draft, or any of those crossed with something else is a good choice.
Also, the saddle fit will be extremely important. When buying a saddle, you want the most and even distribution you can get. The more area, the lower the PSI for the horse and the more he can carry. Any area that might not fit right will be magnified with a heavy rider and can cause trouble later.