Lego Explorers' Arctic Polar Express Train

Excited for your son! This Arctic train a great build and super fun to play with. Unlike a lot of the Lego battery-operated builds, this one requires no phone connection - you can run the the train just with the included remote.

If you add some extra track you can really get some fun things happening (pro tip: put the track out in your garden and let the train roam around under leaves and branches, just make sure to bring everything in before it gets covered in dew or rain).

I'm recommending this train in particular as it sounds like your child loves adventure-themed sets.

If you're looking at trains, the other one to consider is the passenger train, also a super fun build with lots of opportunity for play after it's built.