My son is 5 and has been around trains most of his life. Every year we have had at least one train up under a Christmas tree. As he has gotten older more trees seem to have more trains and trains pop up through out the year. This year he got the lionel Toy Story set from grandparents for his birthday and that has been up in his room since. I also finally built a table for a permanent layout. He just seems to love every bit of it.
I will say, it sometimes surprises me on what keeps his attention. For example, today we added one bench and 5 people to a station and just trying to follow his thought process was fun to watch.
From my experience the remotes are nice, he will control trains with a transformer but he likes to be closer to what he is trying to do or sit in the middle of the track when we had it on the floor.
The only operating car we have is moe and joe which unloads lumber, and he really enjoys that. I don't think you can go wrong with any operating car or accessory.
Uncoupling and/or operating tracks have been a pretty big hit. He is very into reorganizing cars on a train, this just gets better with switch tracks.
Two mainlines has also been quite nice he is into trains passing each other and "dad you control this one and I will drive this one, but honk when you pass me" or "stop so the people can move from my train to yours."
As others have said just keep him involved. If you have a plan talk it through with him, take him to the store to help pick up or out stuff and let him help try to set it up.
