I mentioned the Timeline series of games in a post a while ago, and they’re great for travel because they’re rather small size. Adults and kids both like them, and they play well at 2, 3, or 4 player. Cardline is the same concept, but with different types of topics. The one rub here, though, is they’re out of print. But they’re usually not hard to find on ebay or in the BGG geekmarket. I found mine that way for fairly cheap.
If your son likes pirates, Take the Gold is a quick, small, fun game that involves bluffing and a kraken. The box has it at age 7 and up, but I’m sure your son would have no problems with it. And it plays well at 2 players and up to 6.
Another small one is Ninjitsu, which plays well at 2 and more. It’s about ninjas, and it also has fun bluffing and stealing mechanics. It’s easy to learn and fun–you can check the reviews.
For something more advanced than those and totally suitable for adults as well, Seven Dragons is great. It’s cards, but it’s also building a tableau puzzle. Beautiful art too, especially if your son already likes dragons. We play this often with my wife’s parents.
Those are all card games. For board games, your son might be ready for Catan Jr. or Ticket to Ride Jr. Neither of those are small box, though. He might be ready for Cartagena, which is a fantastic game for adults and kids. I’ve played this for many years with only adults, and it’s always a hit. But I’ve seen kids play too, and it works great at lower ages. My old edition box says ages 8 and up, the new edition box says 13 and up, which is absurd, and the one review on amazon says 6 and up. I would say age 6 to 8, depending on the kid. BGG seems to agree that younger ages are fine with this. A great game on its own and a great filler too, it’s one of the most-played games in my collection.
I don’t know of other small-box games for younger ages, so I’ll be watching for recommendations too : )