If your husband is just not into cycling it is going to be a challenge because it takes time to build fitness. As a matter of fact the burning thighs issue never goes away if one is pushing those higher intensity levels. However, as fitness improves it all becomes more enjoyable at a recreational level.
I come from a powerlifting/bodybuilding background. My legs were trained for over 30 years to have explosive strength at very low repititions. My legs would literally just about be at threshold walking to the water fountain in the gym. For all those years I kept cardio as low as possible in order to win in lifting so the first year in cycling was extremely discomforting to me. The first few miles of those short rides was almost unbearable in the burning sensation so I know what he may be experiencing.
Despite the extreme discomfort, I really wanted to cycle. Not only that but I was told by my doctor that with the blood pressure that I had it would be a matter of months or a couple more years that I would have a stroke. Those two things kept me motivated to continue. But like any type of training it takes personal desire. I used to train people and it would be so tiring trying to keep the unmotivated motivated, but there are little things I could drop as seeds of thought. For me the only seed of thought was the doctor saying, "you are destined for a stroke" and seeing my mother go through one was enough for the seed to take root. I knew at that point I needed some cardio work and cycling seemed to be the avenue since I was too heavy with muscle to run.
Do I still experience discomfort? Yes, when I am pushing the higher training intensities, but if I go out and do a casual 100 mile ride I feel no burning at all.
I am not sure if there is anything from my post that may help you sell the idea of cycling. Hopefully you can find a way to keep him motivated. Pick routes that are easier like a flat route during this initial stage of building the base of fitness may be less discouraging.