When I had a broken ankle in January, I used the toe end of a woolen sock and used velcro or rubber band to hold it on. If it's a removable cast, it's probable that you can cut the sock to cover just the toe area. It might take a couple of layers (socks) to keep the foot warm. I'm sure you're finding out how important the toes are in maintaining balance. Wish him (and you) a speedy recovery.