Was he ever a sexual individual or did the sex wane over time? What is your approach like? Do you try to initiate sex, he refuses, then do you hash it out right then or shortly after?
I know it's easier to talk to me when it's not happening, or just happened. Since you've mentioned your husband likes projects (so do I) try to talk to him about it while you guys are working on the project. It's something of a distraction so your husband doesn't feel so vulnerable and on the spot. It's hard for me to talk about feelings. I'm not a "feeler". I'm horrible at consoling people, being supportive, etc. I'm a "thinker" so I can help you find a solution to your problem. If you approach me in a "thinking" sort of way, 'Hey, listen. I've been doing some research. Perhaps you can be suffering from low testerone. I was thinking maybe we could go to the doctor and get it checked out. What do you think?'
But when approached in a feeling sort of way, it freaks me out. I don't want to talk about it. I already feel bad, but I'm not the best at showing that. I don't do name calling and whatnot, but I would definitely have a hard time hanging around. Sitting down and having a heart to heart is not going to happen it would make me uncomfortable. Maybe change how you approach him about it. It really does make a difference.