Most importantly, read the Paypal policies considering returning your money. I believe that it's a 30-day period after the payment to file a claim that something is afoul. Once this time period expires, you're out of luck. Many scammers know this, and they'll bait you along until it's too late. Give the guy an ultimatum that's a good week or so BEFORE your 30-day period expires, just to be safe. If you hear nothing, pull the trigger.

I bought a car a while back on eBay, and the seller simply never contacted me. I did the same thing I recommended- kept an eye on the calendar, gave him an ultimatum, and then filed a complaint with Paypal. It's not a very quick process; they wait a certain amount of time after you file the initial complaint to give the other party a chance to respond. In my case, the guy never did anything, so I sat around for however long it was (on the order of weeks) and waited until it was finally resolved by Paypal. I got all of my money back.

I'm sorry to hear the transaction was sour, especially right in the middle of the holiday season. I'm sure your son will understand, and that you'll find another E-Revo soon!