If you can, give him his water hot. Thats what we do in the winter, and it helps keep it from freezing.. Also they do have thermal buckets that help protect it from freezing, and if your using the typical 20 gallon buckets from water you can make a floating thermal lid that keeps the water from freezing, my trainer does that. It's wood and I think styrofoam? She makes them herself I think, I have to ask her how she does it. Either way, the wood part is on top and it floats on the horse's buckets in the stalls, when they want water they just push the lid down with their nose and drink normally, and by having that lid on the water can't form ice on top.
I know she has to sometimes show the new boarders' horses how to drink or that the water is there, but after they figure it out, the horses don't mind and it keeps their water warm enough not to freeze, which I know they'd appreciate if they knew how bad it could be. :P