I'm going to guess that there's a reason you're planning to switch rooms, though I agree with PP that Dr. Seuss is a great idea for a toddler room!
Assuming you are going to make the switch, I'd go with something that will last him at least until early elementary school. You're not going to want to repaint the nursery with two little kids underfoot, KWIM?
So, I'm thinking about things that my 4 y.o. would still like:
Racecars
Sports
Dinosaurs
Legos?
I feel like he's mostly outgrown construction and trains. He LOVES animals but I don't see him wanting them for his room a couple years down the road (too "girly"). Fierce ones like sharks or alligators might be ok, but then I'd worry he might get scared at night when your little "tough guy" isn't quite so tough!
I love the idea of doing another room from children's literature but can't off the top of my head think of anything that he won't outgrow too fast. Eric Carle and Curious George or Beatrix potter come to mind but I don't see a 7 year old boy tolerating that (or even a 5 y.o.!) Anybody have any better ideas?
Could you do another Seuss room with the books for older kids--like the Sneetches and Horton, and Thidwick the good hearted Moose, and the 500 hats of Bartholomew (what's his last name?)?
If you wanted to do a Disney theme there are some good choices for a boy. Cars comes to mind.
Have a great time with it!!
My sweet boy 8/06
My beautiful girl 10/09