Hi, I too have a functional tremor in my right/dominant hand and disabilities similar to your husband's, to which i could add a few more, most notably my primary hobby, golf, or certain aspects of it like putting.

Here are some possible solutions I have been watching and conferring with my care team about:
DBS. In your husband's case this was only done once, but it can be performed on both sides to target bilateral symptoms. I assume in his case this was ruled out for some reason?

High intensity focused ultrasound, or HIFU. This relatively new procedure is being aggressively promoted by the manufacturer, Insightec so you can learn all about it on line.

And finally, the "Parkinson's Glove" a very interesting device currently in early clinical trials, I believe at Stanford. This would be the least invasive of all if proven effective. There is a remarkable video on line showing truly amazing results in a single patient. It was dramatic enough, if unscientific, to encourage me to wait to learn more about it, which so far has proven difficult.

Good luck to you and your husband in this quest, and please post anything else you learn.