I think your first step is to write him a firmly worded letter that states that he has not overpaid. Include copies of receipts or other proof that you have and also reference the section of the court order that specifies child expenses and SS. Pretend you are a L when you write the letter. At this point you have seen enough legalease to be able to fake it. Send it certified with a signature receipt request so you are 100% sure he gets it. That way he cannot claim he never got it.
Then he will either pay or he will not. If he does, that means that you showed him you are not a doormat and he cannot under pay just because. If he does not pay then that shows he has no respect for a court order so expect more crap in the future.
I do not think that $400 is enough to be worthy of filing for contempt because it will cost more than that in lawyer fees along. However I would follow crisp's advice and at a minimum call the court clerk to determine how much you would have to pay in court fees if you filed your own contempt order. But if he does not pay then he will do this again. I think once it gets over $1000 would be the time to act.
And from this point forward I think you need to go overboard with documentation and records. make multiple copies of all receipts, and make sure you always get a receipt. And make sure you document all requests for payment of the expenses, either in email with scanned copies of the receipts attached or thru certified letters.