Something like mentioned above, or a quality dry food kibble.
Also with the bathing...I think you should continue to get the hydrobathing done, he will end up being used to it and start enjoying the massage. Have a bucket of hot water ready when the lady arrives and get her to mix it in with the cold water...just to make the bath nice, warm and relaxing for him. The baths do have heaters, but they dont have enough time to warm up. And give him a good treat afterwards (bone, walk, play fetch ect)