Now, generally, all moneys not spend on child, but saved, are his and he gets them once he is 18. So, what needs to be determined is if this 579 account is holding moneys that have been spent on him (and not still yours to use) or if not, are they now HIS. THis seems to be a gray area and I don't see much SSA speak on this, but I'll see if any new word has been provided.
I'd add: You did a tremendous act by saving all his money for him in that 529. A true loving parent gesture - and depending on the child, lots of 18 year olds WILL blow the money if given access at 18.
I am going to see if there is word on whether a spending on the $ into a 529 MEANS there is NO unspent moneys - in which case you tell the SSA there is no unspent moneys because all WERE spent on him. (There are 2 lines of thought: 1) you used the SSA moneys as part of his share of the mortgage/rent/food/clothing/untilities/leisure activities for him, etc. AND the 529 moneys were YOUR other moneys that you graciously used for his 529. THus, his was SPENT on his needs when received --- non left......OR 2) As they came in they WERE spent for him after his needs were met, by putting them into a 529 and that belongs to him, not you (legally, though, that latter part may not be true - we'll see.)
I tend to think OPTION #1 is the "simplest" -- unless you filled out SSA reports over the years saying you were putting his moneys into a 529 - did you have to fill out reports on his moneys and how they were spent? Let me know!