Don't worry, it doesn't matter that he doesnt' understand how sore he will be - kids live in the moment! Its more for us, I sometimes think
If the hospital has TVs for each bed (HDU does, where dd went) that will be a godsend...if not, I would take a portable DVD player with headphones, if you can. Apart from that, I took a couple of books, and a bag with tiny new pressies in (like stocking fillers, tiny toy farm, finger puppet, etc) but tbh, TV and books were the best as she was too sore/tired to play much.
For you, bring a couple of clean tops in case he throws up on you after anaesthetic - quite common! And a couple of short-sleeved ones for him, too. Its warm in hospital, you'll both get hot and sweaty, probably. I also took a bottle of water for me, a drink for her (once she was allowed, post op), some snacks for both of you. Wipes, a few toiletries, a flannel to wipe hot/sore/sicky faces/necks with. Some cash for drinks/phone calls/car park.
If I think of anything else, I'll add later! Meantime, good luck and hang in there - life will get a lot easier in a week or two