I had problems with stool being too watery and getting dehydrated also.
My stoma nurse said that if you have little or no fiber in your diet, this can cause output to be too watery (also, for normal people who are constipated, fiber can help soften stool). She said basically fiber helps the problem either way, to thicken or thin depending on what your body needs.
I began to eat more fiber (cooked carrots work best for me, and whole wheat bread), and my output began to thicken up a bit. I would recommend foods with fiber like cooked peas, carrots, etc. that are more "mushy" then, say, lettuce.
I hope you find a solution that works for your father...I know how frustrating it can be.