Acoustics can be a little harder to play, because the strings tend to be thicker and therefore need more finger pressure to make chords, especially barre chords. Some may argue that this is the best type to start out with as, once he's mastered this, an electric with be comparatively easier. Cheap electrics with tremelo arms often go out of tune easily as the tremelo arm mechanism can cause this. Also there may be a tendency for more broken strings from the outset, as most kids can't resist playing with the tremolo arm a little forcefully.
I started with an acoustic and I would personally recommend starting with this. If you do get an electric with a tremolo arm, take the tremelo arm off until he has mastered the basics.
Don't forget an electric will need an amp and lead
Buy the best that you can afford. Some good makes to look out for are Yamaha, Ibanez, Epiphone and Fender. These are the makes that I know, others may suggest different manufacturers.
As for a full sized guitar, go to a music shop and get him to try one. Also try different makes and get advice from the proprieter. Ask friends etc if they know of a reputable music shop that can help and maybe even offer lessons if he wants them.
Above all, it's supposed to be enjoyable. There are lots of sites online that can show him basic chords and tabs and how things are played. If he learns one or two very basic songs first, this will probably inspire him to learn some of the harder stuff and not give up.
Good luck