Dear Sheetal,
You have written that "He met the gm and requested for him to take him back.". But then what GM told him? Will his services be continued or discontinued?
Secondly, howsoever strong worded letter you might write, I recommend you sending it under the signature of your GM. Bypassing you, when worker approached the GM, you were left in lurch as such. If you issue any letter and if the worker refuses to accept it, you will be left in lurch once again. Therefore, it is advisable to keep GM in the loop and take action accordingly.
Lastly, talk to GM and find out whether he is ready to award the worker any punishment for his unauthorised absence. This punishment is on and above forfeiture of the wages for the absentee period.
Thanks,
Dinesh Divekar