I hate to even say it, because I'm pretty much of an Apple loyalist, but if it's just for music, and if he's not going to be carrying it on his person, an iPod is probably not the best choice.
My DH has an iPhone, but he still uses his Zune for music. You can get a lot more storage for less money on the Zune. The downside to the Zune is that it's large and heavy and doesn't have all the PDA features.
I love my iPod Touch, but (1) I got it for free and (2) I use it for everything, in particular to replace the Palm PDA I used to have. Before I got the Touch, I had a 2nd (maybe 3rd, IAE non-video) generation Nano, which does have the same click-wheel controller as the Classic.
Now, if you want to be able to sync it with the same iTunes Library that you use for your iPods, and/or if all his music is already in iTunes, then that's different.
To use another device's speakers, it needs to have an input for an external audio source. You connect a 1/8" male-male audio cable to the iPod's speaker output and to your other device's input. The last time I bought a boom-box was over 10 years ago, but back then, not all of them had external input. A full-size stereo will have some kind of input, and you can use an adapter if it's a different type of jack. Most modern cars have a MP3 player jack, but if his car is more than about 5 years old, it may not. I wouldn't think it would be hard for a car stereo place to install one, though.