There could be many reasons why he finds sex either a stressful or negative experience. Without him opening up, it’ll be hard for you to work this out. However, it’d definitely be worthwhile to find out what he enjoys about masturbation. It may be that he’s embarrassed to tell you the sort of stimulation he likes, so he relies on masturbation for sexual release.
Tell him you’d love to learn how to pleasure him just as well as he does to himself. Take it slowly and make this suggestion when you’re both relaxed. At first, he might feel a bit nervous showing you what he does. Hopefully he’ll learn to relax and realize you simply want to please him.
Once you get him to open up, you can let him know the sort of touch and stimulation you like too. If this doesn’t work, I suggest marriage counseling.