My son has a billion Pokemon cards, he wants to sell them, are they worth anything really?

Some are, most aren't.
Have a trawl through eBay for ideas.

This website gives the estimated price for pokemon cards.
Pokemon Single Cards - Pokemon Cards (http://www.pokeorder.com/posica.html)

It depends on the card. If they're really rare than they'll be sold for a lot of money. If they aren't then they're just garbage

Agreed, even the ones with little value, if you put 20 or 26 in a stack, you can get something. Most are not worth a lot. If you can go to convention where there is trading, you may do better.