Unfortunately, this is more common than many realize. Mine was also the result of being on antibiotics several times in a short period of time. Clindamycin was the final straw, and a long battle ensued after that. Antibiotics serve their purpose when used properly, but they do have the huge downside of wiping out our bodies own defense system in the gut. I credit the Florastor for finally helping my insides begin to heal.
The symptoms ... lets just say that, if this happens to you, you may want to put a long novel in the bathroom because you will be in there a lot. It is much worse than the typical bout with diarrhea (can I say that here?)
One other thing - make sure he's getting enough fluids. Dehydration can be a problem, especially if he's not feeling like eating or drinking. Make sure he does anyway.
My dad is 97, so I understand a lot of what you are going through. His repeated infections are usually urinary related since he has an indewelling catheter. It's a non-stop battle. I hope everything turns out for the best with your dad - and everyone who is helping with his care.