Okay, a more serious side to this post...
It is the end of the year... I would let it go... Maybe this teacher is a bit out of line and demanding and inconsistant, but it wouldn't really kill your son to go ahead and serve detention and maybe try to be better at remembering things.
Now, on the more serious side.
Having a son with some moderate neurological learning-disabilty, and having a learned a LOT about these kinds of things.
I think it is entirely inappropriate, in general, for schools and teachers to expect 11 year old boys to have the organizational skills and Executive Function skills that some schools and teachers expect them to have. (these are skills that are often deficits with my sons disability)
I just don't find this realistic.
(But, hey the 'experts' in our educational system don't seem to recognise this

)
This is one problem that I see with the common practice of moving kids who are just now turning 11 to 'middle school'.
I know that many 12-13-14 year olds (especially boys, it seems) can have troubles with this.
They can walk into class with a completed assignment, and just 'forget' to turn it in.
So, while I would advise you to just let-it-go at this point, at the end of the year. KNOW THAT I AM COMPLETELY WITH YOU IN YOUR FRUSTRATION ABOUT THIS WHOLE THING!!!!!