Yeah, he's a full time liar. He sent it not his son. Agree 100% with Frizz....
First, your first message shouldn't have threatened negative feedback. The seller can use that against you for "feedback extortion" and nullify the auction and get his fees back and prevent it from hitting him. (Seriously--the worse the seller the more they seem to be able to bend eBay rules with enough followup that good sellers can't get from them.)
Second, your second message should not be questioning his motives, etc.. Keep it short, sweet and professional. No accusations, just pretend to go along with his lies and confirm his address and say you'll be there to pick up. (Yes, you can still clarify that you were confused by his 'sons' mesage.)
Yeah, you know the real story, but if there is still any chance of getting what you want, be sure you keep it professional and brief. Share the experience afterwards, don't poke at the idiot when you still want something from them.
Also, you might wanna take a friend with to assist. If he actually relinquishes the game to you he certainly isn't going to be happy to help you load it and might help damage it even while you're there. Also, you don't know what this person might/can do for threats and the like. Make sure you have someone with to even things up just in case something goes worse. Watch a couple episodes of Operation Repro before heading over alone and you'll see what I mean.