Can you post the auction number?
When you leave negative feedback, be factual and do not get personal or insulting toward the other party. That way, if someone later does their research and sees that you left a calm and factual comment and they didn't, it looks worse on the other party.
Before doing anything (even leaving feedback), contact the seller. You can get the seller's contact information (name, address, phone #) through eBay. Since you had a transaction with this person, I'm surprised you didn't get it anyway. Give them another chance to make it right. Chances are they may, but chances are they may not.
If they aren't willing to resolve it, then take further action. If you paid by Paypal, file a complaint through them. Also, file a dispute through eBay. If they advertised actual Yu-Gi-Oh cards but sent fake ones, it sounds like you have a case for an "Item not as described" dispute. I've never gone through one of those, so I can't talk you through it. Details for that should be on eBay.
If the seller agrees to a refund, they may insist on the item being returned before refunding money. Did your son open the packs? If so, they may not refund since the cards won't be in their original condition. In that case, file with Paypal (if that's how you paid) and leave negative feedback. If they do agree to refund, make sure you send with delivery confirmation. That way if they try to say they never received the item, DC will tell you that it was delivered.
I'm sorry you and your son are having to deal with this. Good luck!