L.D.
My son has much the same issues so I understand how hard it is to explain things sometimes with a child who has a harder time understanding the finer nuances of certain social rules.
I guess, for me, I would probably back off from using the word "exception" in discussing things with him if, in fact, you are using that term -- that may be too confusing of a term for him at this time -- and I would break up concepts that you are teaching him into smaller, easier to understand modules. Let me explain . . .
With the example that you had given about the show & tell toy and the book, I would probably break that into two separate rules and not co-mingle the rules. For example, one of the rules that you son has is that he can only bring a toy to school for show & tell on his show & tell day. Another rule (separate from the first rule) is that when a friend or a teacher asks to borrow something specific like a book from you and mom and/or dad says it's okay, then you can take that item to class to let your friend or teacher borrow it on a non-show & tell day.
Hope this helps.