Is he 504 eligible? That is, have they found him eligible?
If not-- you've got nothing at the moment, in terms of leverage.
And NO, I would not just let this go.
But think of it this way-- that disability process MUST work forward to back or the school would have every parent breathing down their necks to "adjust" things that their precious snowflakes simply find unpleasant, hard, or unrewarding (to either parents or child). There's a big difference between "I have no readiness or ability for this level of material that is 'advanced' relative to grade, and it's going to be a CONTINUOUS battle to get me through it" and "I can't WRITE my answers or they don't make sense-- I need another way of getting what I know across to those evaluating assessments because I know a LOT if you give me a chance."
The latter is disability. The former is special-snowflake-ism. Schools SHOULD ignore the former, and should at the same time find out ways to accommodate the latter so that those students have the same opportunity to LEARN at school as other children.
But the first step is teasing apart who is who.
"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.)
