VAT late registration - is there anyway to help my client?

My client should have registered for VAT in April but we have only just registered him now.  He made £20k of sales in June which he will be liable to pay output VAT on.  Is there anyway that he can charge this to his own customer on a separate invoice (they are VAT registered so they could claim it straight back) or should he do a credit and re-bill, or is it too late to do anything about it?