yes i would say the apple shuffle , i have one its so easy to work and clips on so not to lose, got my mom one who is in the middle of alz and its working great for her.
I would suggest the Apple Ipod Shuffle 2nd generation, the Sansa Clip is another (but it has a screen on it, so it may confuse him). The shuffle has no screen, but the controls on the front are pretty easy to understand. There are even less expensive models that are USB compatible. They have very basic controls.
I am sorry to hear about your Dad and I am afriad I dont know much about the MP3 player other than they are good, I am hopingothers here may know but we also have an expert /Doctor forum and the Doctor is very interested I think he may have some input for you, so go back to forums check out alzheimers forum on the right side wher it says epert Forum . Good Luck I hope he does well.