Becky,
Sounds like your son is into physics and possibly architecture. Sandbox play, building ramps and roads and such is still science. Might want to look in your science experiment book for some experiments for physics stuff-levers, pulleys, ramps, friction. Go with what he enjoys and his interests will expand as he grows and you can move on then.
You could even teach him math in the sandbox-build 4 roads or if I build 2 roads and you build 3 how many are there.
He is still quite young so don’t feel like you have to push him. Short bursts of reading are fine. Make sure you include reading about things he likes-trucks, trains, construction vehicles. You might want to check out Macaulay’s books on castles, cathedrals, etc just for the pictures-you could read them if he wants but the pictures alone are great and would give a lot of information on building stuff especially if you just gave him some info to go with it (you pre-read and then “narrate” back to him what he’s be interested in). Field trips: drive by a construction site and stop to watch (lots of physics there!), go to a train depot or even on a train ride so he can really get a look at them, go to a fire station or police station.
My eldest boy was like this when he was little. We just worked with it. He is now a science and history buff. He may always be interested in cars and such but he will also show interest in other things too. Enjoy it!
-Rebecca