I also have one foot that's incredibly cold at times. First, make sure that there's a pulse in his feet. I had some tests done and they decided that there was some sort of vascular spasms... since I'm not a doc, not sure exactly. But I do take a pill each morning which helps the circulatory system, and I guess also lowers blood pressure, which wasn't an issue with me.
Day to day warmth... soak in warm water, and wrap warm wet towels around foot to warm up... be careful not too hot. Keep foot elevated as much as possible. Get one of those machines that you stick your feet into to massage/jiggle them to increase circulation. I used to use the aromatic pillows that you put in the microwave, but broke down and just bought a small heating pad that I wrap around my feet at night.
I also took advice from Ann, who told me that she wore sheepskin lined boots year round, which I've been doing... at least until it hits 90°! I also use magnet insoles in my shoes which helps the swelling to stay down. As I sit here typing, I'm heading for bed in a few minutes to try to warm up my feet!