Client Charged GST Before He Was GST Registered

I have a new client that is way, way behind in everything.  In fact, no bookkeeping has been done and no CRA filings have been done.  He started charging GST on his invoices from day one, but did not register for GST until several months later.  What happens to the GST he charged before he registered?  Does he need to change his GST filing start date?  I haven't done any posting yet so need to know the best way to deal with this.

Thanks for your help.