Sweet! You know I'm going to have some unsolicited advice for you.
Tomorrow is the annual bridge walk on the Mac bridge. I'm not sure what your schedule is like but I would try to avoid it, it will be a disaster.
Check out Castle Rock on St. Ignace on the UP side of the bridge. It's a hokey tourist trap but your boy will probably dig taking a picture with the giant Paul Bunyan and climbing up to the lookout. The local delicacy is a pasty. It's just a meat pie with gravy and vegetables but people love them up there for some reason.
Avoid Mackinaw City. The island is pretty touristy and not my favorite thing and Mac City is like a cheaper more touristy version but without horses.
The best part of the State imo is the Northern lower in the summer. Petoskey, Chalevoix, Traverse City, Torch Lake, Harbor Springs, Glen Arbor all beautiful. Stuff for kids to do in most of the above. Shorts brewery is in Bellaire on Torch Lake. Dockside is a burger bar with a nice patio on the lake and fun to watch the boats pull up and go by.
There is an indoor water park and new giant Sky bridge with a view at Boyne Mountain. You might already see some fall colors this time of year.
If you only have time to do one stop I would do Traverse City or Petoskey. Both great towns with plenty to do. Traverse City has a better beach and beach facilities. There is also a strip in Acme on your way into TC that has hokey shit for the kids. Pirate's Cove Adventure golf, zip lines etc. Don's drive inn for burgers and milkshakes.
Oh and Sleeping Bear dunes. You will be shocked that Michigan has scenery that looks like that.
Have fun!