First, welcome to the WhatNext forum.
I believe that the best place to start is your dad's primary care physician. Is your dad as concerned about what has been happening as you are? If so, just encourage him to go get a check-up and tell the doc about what has been happening. Ask for a referral to a specialist if anything can be pinned down. If he is not concerned then there isn't much you can do. He is an adult and you don't want to nag.