Actually, you don't have to 'forward' his mail. On each envelope write:
"NO LONGER AT THIS ADDRESS, RETURN TO SENDER."
Eventually, when he doesn't get his mail, you are NOT a forwarding service, he will take the time to go to the 'important' places (ie bank) and change his address.
The Post Office is very obliging about returning mail to the sender.
One less reason for you to have any contact, even through his parents.
It is not your fault that he has been too lazy, to put in a change of address.
J M H O
Love and hug,