You have done a great job on that layout!
Many years ago, when my son was small, we built an HO layout on a 4x8 sheet of plywood. He also liked cars, so we had a lot of roads, a couple of tunnels and a few buildings.
Last year, he built a similar layout with his 4 year old son. The next time they paid us a visit (they live a few hundred miles away), my grandson wanted to know why there weren't any trains at my house!
So, I started an N scale layout - also 4x8. When he was here last summer, the track was laid and a few roadways were down. He liked the layout, but made it clear that the next time he came he expected to see a lot more trains and a lot more cars!
So, I'll bet that someday he will be building a layout with his son, and you'll have another generation of model railroaders.
I started my layout for my grandson, but I think I am enjoying it even more than he will!