There are a lot of people who advise learning to play on your typical steel string dreadnought acoustic guitar. I'm not one of them. Acoustic guitars have higher action, and thicker strings. The most common shape (the dreadnought) forces the you to hold it at certain angles that can be pretty far from ergonomic. Typically they've got smaller frets too, although that's not universal. This means that your finger tips will get sore faster, you're more likely to be uncomfy from the way the guitar body presses into you, and your hands will hurt more than learning on a nylon string classical guitar or an electric. Less pain means more practice which means quicker progression, less frustration, and greater likelihood to stick with things.
All that can be overcome if someone is determined enough and really loves the sound of their instrument though. What kind of music does your son like to listen to? You need to involve him in this decision since he's going to be the guy playing it.
OK, back to attempting to answer your question:
Your best bet is to find someone who can play a little to try it out and give you some feedback on a used guitar purchased privately. Strumming a few chords, tuning the guitar, doing a few single note runs, checking the action, checking the intonation, all of these things will take seconds for someone who is used to playing and are vitally important to getting a good guitar.
Obvious stuff that anyone can check for:
- The neck should not be straight and not curved up or down towards the tuning pegs. (A little bow forward towards the body of the guitar is OK, and should be present on a well set up guitar.)
- Look for fret wear (little divots on the frets under the strings) because this can make it impossible to play notes cleanly
- Speaking of clean notes, it should be possible to fret each note on the fretboard without hearing any buzzing (and without a tremendous amount of finger effort). A nice clean note should ring out every time.
- No buckling around the bridge area on the top of the guitar (sometimes happens when someone puts strings that are too heavy on an acoustic, happens often on a 12 string acoustic).
As far as guitar brands, I've played good guitars and real stinkers from most of the major manufacturers. You want something that plays well and that sounds good to the guy who's going to be playing it.
About a 500$ price range will give you lots of options for excellent quality instruments.
