I know someone who is the same way, absolutely no sex drive. This does not mean he is not caring or affectionate, he just has no sex drive, he told me so. However this is untrue, his sex drive is unaturally high, he just doesn't know it. He is a martial artist and takes all of his aggressions out in training, which leaves him feeling better.
You see, sex in men is driven by...well...anger. In med. science, they say that it is testosterone that makes men aggressive, also it's that pent up aggression/testosterone that creates our sex drive, and makes it ultra high.
So your husband may be lacking in testosterone. This does not mean that he sucks in bed or anything, it just makes him less inclined to want sex. Fortunately, there is a solution (if this is the case) He can join the gym and work out, which helps boost sex drive, also, renting violent movies would help, not sex related, action movies. It's in us to be violent, and those of us guys who are not turned on by watching one guy beating down on another, well...are not guys.
Anyways, try it out, see if it works.