Well, your dad obviously likes artists with beautiful voices. Some fantastic artists and albums you may want to try are:
Eva CassidyLive at Blues Alley. Eva's great because, well, her voice is one of the most beautiful female voices around. Also, she covers well known tunes -- you know, the tunes that your dad can sing along with.
Diana KrallAll for You: Dedication to Nat King Cole. Again, another beautiful voice -- more on the subtle jazzy tip, and the bonus is, again, more familiar tunes. Not sure how old your pop is, but most people 40+ know more than a few Nat King Cole songs.