Hey Kristi,
You can get what are called theme decks at Target (they're in an aisle at the front of the store by the cash registers usually with other trading card games). These "decks" will usually have a couple of rarer cards and a bunch of common cards, but ultimately, they should include a good set of cards that are built to work well together and will allow your son to dive in and play a game with his friends.
That said, if your son ends up loving it and getting into it, he'll probably want you to buy something called booster packs. These are small, addictive packs of cards which include 15 cards that normally will include one rare card, a couple of uncommon cards and a bunch of common cards. They are NOT built to work together, but rather to boost the collectable nature of the game as you open packs to try to collect the cards you need to make your "deck" better.
It's a slippery slope of fun that can get expensive fast, but if it were me, I'd probably buy a theme deck or two and 4-5 booster packs so he can have the pokemon experience for Christmas. He may love it, he may get bored, but you should be able to get all of that for <$50.
Let me know if you have any other questions and I'll try to help!
Robin