Go to Atlas O website at Atlas O dot com and click on the link "Layout and Track"...
Then click on "For Atlas O Layout Gallery click Here" and "click there"...
Next click on the "O-3 Rail Nickel Silver Layout Gallery". and...yep, click that too.
Now, you should see rough sketches of track plans. Pick one you like and click on that bad boy too. You should see a list of the necessary components to build that layout.
You CAN use other track from other manufacturers...but that may involve some adapting of curves or switches or length of track pieces. That is all part of the fun and challenge of the hobby. You can run nearly every 3 rail compatible model or toy train on that track, so it would be rare that the trains and track cannot be used together UNLESS you opt for 2 rail track. And while it may not be obvious, if you want to run large or long modern rail cars and locomotives on tight radius curves, you might find that double track curves are a problem as the cars may sideswipe each other.
One of the best things you can do is go to a real hobby store and speak to the owner. They would love to make sure your experience in the hobby for both you and your grandson is a positive experience. Likewise, you can find a model railroad club in your area. (I'm from north central Oklahoma...probably no where near you...I'm not even near myself sometimes.) Clubs often exist partially to educate and help beginners in the hobby, so see if you can track one down in your area.
I hope you and your grandson have many hours of fun with model trains. Enjoy.