Gst input credit

If he was not registered earlier and on Registration date under GST 

In his stock include goods on which he paid VAT & Excise Duty, 

On the Invoice basis he can claimed vat & excise amount ( after filing GST TRN-1) whithin prescribe time limite

yes......if you have invoices (must not be older than 12 months), than you will get full credit of vat and excise duty paid on the purchases which are still lying in your stock......
if somehow you misplaced the invoices, than you can take credit of 60 % of the value @ the prevailing gst rates if gst rate is 18% or more, otherwise 40%....(for example if value of goods is 100000 and rate of gst is 18%, you will get credit of RS 5400 as cgst and 5400 as sgst/utgst.

to avail this credit file gst trn 1 before 90 days from 1st July.....

Thanks a lot for detailed reply