"The work is super-easy, so he's getting by" is a clear warning sign in my mind. You can deal with it now, or you can remediate fall-out in a year or two. IMO, you're better off getting the 504 plan in place NOW than waiting until he's in crisis and you're all frustrated and anxious to move things faster than they can reasonably go. (Schools move slowly on this sort of thing.)
ITA. Do you have a written eval from a neuropsychologist? Have you talked to the district-level people, or just the school?
There are things you can do to make a 504 happen even with a recalcitrant school.
