Most playmats don't have the play zones laid out unless you get a custom one made, but you could get something like this. $20 isn't bad, and it's 2 players.
http://www.ebay.com/itm/Pokemon-TCG...137975?hash=item33c57567f7:g:NFEAAOSwJ7RYUJiJ
For storage, the cheapest option is a cardboard box from any gamestore for a couple of bucks, and a cheap .99 cent binder and some card holder sheets from a game stop or walmart for his rare and favorite cards. If you can find them you can also get a mini binder that comes with a pack for $5 like this http://www.coolstuffinc.com/p/236736.
As for what to buy next, well I assume you're just going to be playing at home, so honestly just buy whatever makes him happy. Theme decks will ensure you know what you're getting and are all pretty simple to play. Boosters are more fun to open though. The Keldeo vs Rayquaza battle arena decks are also really good, but might be a little complex for him. They're a step up from theme decks, but not as complex as actual competitive decks. But they come with 2 decks that are pretty balanced against each other, with way better cards than the theme decks.
If he was older and you guys were looking to get into competitive play I would say to stick to singles only, as you'll never get the cards necessary to build any decent deck just relying on booster packs, but even with older kids they just love opening boosters too much. For the same $4 you can get a booster and get garbage, or buy a top tier card like Yveltal EX, and kids will prefer the booster every time.
