By law, he needs to reissue all incorrect VAT invoices, amend all affected returns and claim a refund from Revenue (He has paid over 21% instead of 13.5%).
You will need to amend all the affected VAT returns and pay any amount o/s (you have claimed 21% instead of 13.5%).
Obviously, your supplier will owe you a similiar amount (to that which you owe Revenue).
By doing this, Revenue attention will be drawn and it is very likely that your supplier will be audited.
Remember, don't do any adjustments on your side without valid amended purchase invoices. If he gives you any problems, just give his VAT number to Revenue.