If the factory FM radio is still working, get one that is FM modulated. This type of player will plug into the radio via the antenna jack and your antenna will plug in behind the player. The player will play the disc and convert the digital info on the disc to an FM signal. You will set a frequency on the player and whenever you want to listen to the CDs you just push play and dial it in on the radio. Usually you pick a frequency that isn't used in your area or one down low on the dial where stations are typically weak.
I have been happy with my FM modulated JVC 12-disc changer. I think you'll be satisfied with any major brand name. Depending on how high-tech your son is and how big his music collection is, you may want to get a player that plays MP3 files. I wish mine was a MP3 player, but I've had it since before MP3s came into fashion. Check out satellite radios too while you're at it. If you're out in the country, you may like that option as well.