Teresa,
I don't have any advice as to why your Dad's tumor appears to have shrunk on its own. But it's good news, so that's something to be thankful for. You may also find that the chemo is not as bad as you (and he) fears. My mom was pleasantly surprised that it was not as hard as she thought it was going to be. She felt tired and achy for a few days afterwards, but bounced back fairly quickly. With the exception of the 3-4 days immediatly following her chemo she kept her normal schedule.
Good luck to your Dad and remember that we are here if you need us.
Susan