I'm sure you'd have no problem if it has the stock exhaust.. All he needs is an ORV sticker.
The DNR I've run into up there are pretty nice and not out looking to see what they can ticket you for.
Technically he would need to be 12 years old, under direct adult supervision and possess a ORV safety certificate.
I don't think you'd have an issue with him being 11, but not having the ORV safety certificate might get ya a ticket
Now if he was riding a dirt bike there is no age requirement, just direct adult supervision and ORV safety certificate.. lol.
Here is a link with the laws... The pages you'd want to read are 29-32.
http://www.offroad-ed.com/assets/pdf/handbook/mi_handbook_entire.pdf