Mac Book - 6 digit pin code issue?

Bought macbook for my son and then reset the login password by first putting the previous owner password and then creating new one for my son (all done in system pref.), after that i removed previous owner icloud id and left it for a while ,when i returned i find it locked - it screen is asking for 6 digit pin code , previous owner has no knowledge about it and he doesnt have the purchase receipt as well , what shall i do???