I would talk to your vet about hyaluronic acid injections. Ask about the generic version. They really helped my arthritic dog and have done wonders for a few friend's horses. You should be able to give them yourself once your vet shows you how. You give them once a week for 6 weeks, then once every 2 weeks for a while, then once a month. Some animals can go every 6-8 weeks eventually. The injections work better than feed-through hyaluronic acid.
With that I would also treat him with a oral supplement. Look for one that is NASC certified and contains high levels of glucosamine, chondroitin, vitamin C, and MSM. I have had good luck with HorseTech.com products. They have a couple that would fit the bill for your wife's horse. Quick Links View Cart
Other good joint supplements: SmartFlex Senior from SmartPak Equine Senior Flex from SmartPak Equine Equinyl Combo from SmartPak Equine Matrix 4 from SmartPak Equine Next Level Pellets from SmartPak Equine
Try one of these instead of bute next winter: B-L Pellets from SmartPak Equine Devil's Claw Plus from SmartPak Equine
Also, keep riding him! Arthrtic horses do better the more exercise they have.
If boots helped him, then you might want to find a different farrier, one who has experience with arthritic horses. A change in hoof angle, shape, or length may help ease his discomfort even more.