Pokemon TCG

I know nothing about any of this. My 8yr old son has been asking for Pokemon cards for a while now. He doesn't know anything about them either, other than his friends at school have them and apparently they are the absolute coolest things ever. He wants to learn how to play and not just collect the cards.So.. where should I start? I purchased the following boxes from Amazon but I am completely lost: Latias and Latios Trainer Card Game Rayquaza Ex Box Charizard Ex Box Krookodile Ex Box Garchomp Ex Box

... My thought process was that the Trainer Card Game would teach us the basics for HOW to play it and the other boxes would give him cards to start building his deck. I also purchased a Pikachu binder and some sleeves for him to put the cards in but I'm wondering if I should look into something else for storage?

Anyway.. Questions:

1. Are cards compatible with each other? I believe the trainer card game said XY, and the other boxes I got say EX. Does this mean they are different decks that must be played separately or can they all be played together?
2. Do the box sets have the same cards in them? In other words, if I were to buy a second Rayquaza Ex Box, would it have different cards or the same ones? If different, is it good to get multiple boxes of the same one or is it best to get different boxes?
3. Is it better to buy individual booster packs or should I stick with getting the boxes?
4. Any recommendations for the best types of boxes/sets/tins to get? (note that price is a HUGE concern as I am struggling this year. The ones I got were during a really great sale on Amazon and were much cheaper than regular price)

TL;DR - Need help figuring out how to get started with Pokemon TCG for my son.