When I was giving lessons, I found that kids who owned an electric guitar, without knowing how to play it, got more frustrated because they couldn't get it to make the cool sounds they thought they could. They also get distracted by whammy bars, switches, knobs and amps. Acoustics, though entirely less cool, seem to streamline the learning process.
And don't buy used (especially electric). Kids **** guitars up all the time, and you'll want a warranty when the tuning key just happens to fall off or the output jack magically becomes intermittent. I worked for Fender in the tech dept, and we always helped kids out even if the problem was obviously a non warranty issue.
The suggestions on here for PRS's are a bit pricey for a first guitar IMO. 200-300$ and you will have a decent guitar that won't fold in on itself in a year yet cheap enough to gamble that he'll stick with it. And again, have it set up professionally. Pay 100 or whatever to get it setup by the best tech in town. Make sure the nut slots are filed correctly and to the right height. This is probably the most important thing for a first time player. They live in the first position and if the nut slots are too high, it will be really hard to play. I would also recommend (if you decide to go acoustic) try to get one that doesn't have a solid wood top. They sound better, but are prone to drying out and cracking if not properly humidified. Especially this time of year. I have 6 or 7 guitars in my shop right now that are in for cracked tops because the guitar dried out. Some with multiple cracks at 70-140$ a crack to repair depending on whether or not I have to spline them. That's without finish touch up. Three of those guitars are Martins. $$$.
And there's nothing wrong with buying online this time. He probably doesn't have the ear yet to know what sounds "good" and what doesn't. Once he's established the ability to play, as well as a more sensitive palette, take him to a local guitar shop to try them out. See which one likes him best.
Companies to Avoid: Dean, Washburn, Oscar Schmidt, Jasmine, Samick. That's not to say that occasionally they put out a decent guitar, but more often than not, if one of these crosses my bench, it's a POS.
Companies to explore: Yamaha, Fender, Seagull, Epiphone, BlueRidge, Art & Lutherie, Ibanez. These, in general, have better build quality, setup nicer, and more frequently sound better.