Jomarprop, I realise this is not PC, but 300+lbs all going through your lower back and bum, will always be uncomfortable on a motorcycle. There really are 2 things that will help, Stretching/exercise to build up the lower back muscles and loosing weight. I am sorry to hear your husband has so much discomfort, but I would think that would be the case on any motorcycle not just the wing. With that much weight, he needs to really build up the lower back muscles to avoid discomfort and try stretching to get some extra mobility (and muscle tone). A gel seat might also help a bit but I suspect the stretching/exercise will be the best solution but it does require time and effort.
Gary
PS I am 6'2" and used to weigh 280+lbs, I could ride about 300 miles before I had severe back ache and had to stop. I had a lap band fitted 2 years ago and lost 100lbs, I can now ride for 100's of miles and have no back ache at all. I did not stretch or exercise before loosing the weight but know that this much weight will really make riding uncomfortable, a back rest might also help your husband a bit.