Give him a treat everytime he enters the washstall, this makes going into the washstall pleasent for him. Don't wash him with really cold water, make it lukewarm-ish, cause cold water on a hot day just feels cold and unpleasent. Start with his legs, keep with him when he moves. The second he stops moving, take the water off his legs and rub him and tell him he's a good boy. Continue to do this and soon he'll see that the washstall isn't so bad after all :)
Your greatest weakness is God's greatest opportunity to show His strength through your life if you are willing to trust Him.
~Give. Me. Faith. -Elevation Church