Yes, it's difficult to force someone to be safe. I'm probably a little over-cautious, but I worry about my equipment and others as much as myself.
-PTO override and safety chains all around.
-Seatbelt and ROPs since the tractor has them.
-Never leaving the seat with the implement engaged.
-Never crawling under a raised implement or the front-end loader.
Quite honestly, as worried as you sound about this, the idea of "taking away the keys" may just be your most logical option. Sure, I understand the heat it would create, but it sounds like your husband is endangering himself, the investment in the equipment, and the well-being of others like yourself with this mindset.
Also, it sounds like your husband might need a "good influence" or example to spend a little time with him about this. Any neighbors or family willing and knowledgable with equipment? How about the dealer you purchased the equipment from? A sales rep demo of proper procedures and a little sobering instruction in safety and warranties from someone official might get his attention...???