Gst credit of stock in hand

Dear All,

My One of client is actually not cross the specified limt of turnover for eligible to GST registration, but due to continuos asking for GST invoice by some old customer he willing to made voluntarily regular delar registration. Now the problem is presently he has around Rs.6 lakhs stock in hand and if he made registration now, then he has to file return from this month itself.

Now the problem is as he is not register till date his all purchase / stock in hand items invoices is urd invoices, he fall in 18% GST categery so if he register he has to sale in stock material @ 18% but without ITC. 

So what can do in such cases.