I had good luck with Glazen GL from horsetech.com. It's an all-in-one supplements for joints, skin, and coat. They have a bunch of other joint related supplements too. They are very nice to work with and they sell direct to the public, so you know it's fresh. They'll also custom blend you a supplement for no extra charge (just for the ingrediants).
I would recommend getting one that has Glucosamine, Chondroitin, Vitamin C, and either MSM or Yucca. The MSM and Yuccan are anti-inflamitories. I take MSM for myself, and it really helps reduce my sore knees during the winter.
Talk to your vet about giving him a little bute on days you plan on riding him harder than usual, or on days he just looks a bit off.
Also, talk to a farrier about giving him a more natural trim, with lower heels and shorter toes. That can help some horses loosen up quicker and be more comfortable on their feet.
The main thing with arthritic horses though is movement. He should be turned out 24/7, or as much as possible, and should be ridden or worked often. If he's too stiff to ride, take him on a "walk" on the trail. Walking in hand is a lot of fun and a great bonding experience