For varying definitions of “close” - Williamsburg is 2.5 hours in good traffic and us DC-area folks know how rare THAT is
If you were to consider Williamsburg, the historic area can be fun but I’d bet he’d be bored silly; there is also Busch Gardens (theme park) and Water Country USA. Both will be crowded as hell during the summer of course.
As an alternative, especially if you’re driving: Hershey and other areas in central PA. They’re less out of your way to/from DC than Williamsburg would be, and of course CHOCOLATE. The Chocolate World tour is “free” (though they have some add-on experiences that cost) and of course you’re dumped out of the tour into a large area with ever kind of Hershey product (edible and not) you could imagine. And the theme park is nice - I’m biased (as I grew up near it) but I like it better than Busch Gardens and Kings Dominion (and a LOT better than Six Flags, which is about 20 miles east of DC). If you go to Hersheypark, stop at a Giant grocery store in the area for discounted tickets - and if you go into the park after 7 PM (I think that’s the cutoff) that same ticket is good for the next day as well. If there are any mobility issues, Hersheypark is hilly, though they do rent scooters.
Plus Hershey is near Amish country which can be scenic. Depending on where in the Midwest, you may be familiar with the Amish but I gather the Pennsylvania community is a tad more progressive (judging by some of those ex-Amish TV series, I could be wrong) and quite willing to profit from tourism.
There’s also a train museum in Strasbourg, PA, which is fun for a 9 year old for a couple hours. Heading there, we were treated to the sight of an Amish man taking his horse and buggy through a bank’s drive-through lane :).
Harrisburg (near Hershey) has a nice hands-on science museum right downtown (the Whittaker Center) which might be right for a 9 year old. Parking can be a pain, but not as bad as, say, a bigger city.
Do not bother with Dutch Wonderland (small “theme” park in Lancaster); it’s targeted at the younger crowd and your son would be bored silly.
Another destination to visit is Baltimore - the Inner Harbor area has a lot of fun stuff to do and see including the aquarium, a WW2 submarine, an old ship, a science museum, the water taxi is fun, and it’s fun to go to one of the restaurants where they pile your table with crabs which you get to dismember and eat.