Masturbation does not cause ED and I wish people would stop spreading medically incorrect information.
I'm in a similar situation to the OP, but much shorter time span, and we are younger.
It was very difficult to get my husband to see a doctor and I tried to press him gently but firmly for nearly three years before I finally made it clear that I did not want to spend the rest of my life feeling neglected and denied any real passion, that it wasn't what I wanted to see in the future of our marriage. He got the hint, but it's still a struggle to get him to take each subsequent step.
You have to just press on, and stay calm in the face of his defensiveness. He's scared because it's a scary and embarrassing thing for a man to deal with, but don't give up, because if he knows you'll back off, he will happily give up and go back into denial.
My husband rarely masturbates because the ED he's suffering from is making it difficult for him to maintain an erection and to climax. I thought that if he never masturbated, he would find it easier to maintain during sex, but this has not been the case.
We are still not sure whether the issue is more psychological or physiological, but at this point I am trying to stay on top of this without being a nag. And that's all you can do, admit that you're going to have to push him a bit. Just don't give up. And if he doesn't want to give you up, he'll step up.