You will hear that N scale isn’t suited for young kids; in fact, I was just re-reading a recent (last year or two) MR issue giving advise on layout scale sizes repeating this old mantra - HO or bigger is better for young kids. My response - horse hockey!
What size is Brio, Thomas the tank, Harry Potter, and the list goes on, trains for kids : they’re all designed to fit into the hand of a young tyke. If you go to Walmart, Kmart, Target, etc, you will find most toys designed to fit into the hands of a child, not an adult.
With N scale, you need to explain to your son that it is very special and expensive so he needs to look after it - I’m willing to bet he does. In fact, he’ll be better at re-railing cars and engines than you will, very quickly. Trust me, I learned this from experience.
As for a “set”, go to your local quality hobby shop, forthwith known as your LHS, and tell him you’d like to buy the works, engine, a couple of cars, track, a couple of turnouts, etc and ask him if he gives discounts when you buy like that (he does and if he doesn’t, it isn’t a good LHS). 10% is usually a “gimme” discount rate.
If you want to buy a scenery kit that will motivate you to get that layout up and quick (you’ll hear this often, trust me - “Dad, when’s the layout going to done?” - another version of - “are we there yet?”), I’d suggest you buy the Mountain Valley Scenery Kit by Woodland Scenics. Its a sleeper of a kit, designed for a 4 by 8, but so what if your layout is a bit smaller. This kit has everything you will need to get your scenery up, and I mean everything. About the only other thing you will need to buy are the structures.
By the way, about the only thing crappy about the kit, is the picture on the front box cover - the kit is way better than the picture implies.