A few of my friends, who work in industries where they needed a stronger metal (like carpenter’s for instance) for their rings so they wouldn’t get damaged opted for titanium. I think you can also get rings made of stainless steel.
I second the notion of getting a jewelry artist to help you to design your own ring. You can also look in antique stores (if your fiance doesn’t mind a used ring. For me that would be the best option because I love the idea of choosing a ring that already had a history, plus I like the look of the delicately carved older rings from the early 1900’s).
Another option is to take some rings or other jewelry and have them “re-purposed” into what your fiance would like.
Find out what kinds of metals he does like and whether he likes a more minimalist look or whether he prefers something more ornate, or something more artistic in design. Then you can figure it out by process of elimination.
Here is some info on choosing an appropriate men’s wedding band.