Hello and first off let me apologize for this question going so very long without a response. The problem was all your questions were somehow posted in the OB/GYN section instead of the Medical section.
For now certainly what your father needs are plenty of fluids--not just water, but juices, GatorAde, broths, and soups to start with.
As far as more solid food I'd suggest the BRAT diet of bananas, rice applesauce and toast.
If he can tolerate them, Ensure liquid and puddings would be als a good way for him to get some calorie dense nutrition.
Let me know if you need to discuss this any further please