That's incredibly frustrating, especially when you've done everything right and it's for your son's birthday. Sadly, once a gift card shows as "already redeemed," there's nothing other users here can fix from our side. It really does have to go through Meta's internal investigation process.
I know that's the last thing you want to hear when you've already spent time on live chat, but these cases usually do get resolved - they just aren't always quick about it. Hopefully support can verify the issue and reissue the balance soon.
You're definitely not alone in this happening, and you've done everything you can for now. All that's left is waiting for their team to finish the review, as frustrating as that is.
When you do get it resolved, here is a useful tip: if you apply the promo code TNORTHVR at checkout, it will give you a 10% discount on non-sale games and apps.
I hope your son gets his birthday gift soon.