N.L.
My son is 4 and is like your son. In fact I asked a question seeking advice on how I can answers his questions about our solar system.
We bought a telescope for my daughter when she was 5 at Toys R Us. My son has been using it for a couple of years (with my assistance of course).
He LOVES it! I can't remember what brand or anything but I definitely bought it at Toys R Us. We've had it now for 10 years and still works great.
It was a kid's telescope so didn't cost a great deal.
Good luck!