You can write on the front of the envelopes when they arrive:
"Return to Sender"
"No Known Forwarding Address"
Then put the envelopes back in the outgoing mail. And just keep doing this until the envelopes stop coming.
You are not required by any sense of obligation to continue to be his mail processing station and his cover for his PO. I am also concerned that the more you participate in his deception, the more you may be held responsible by his PO for the collaboration in the deception.
You've gotten a lot of great advice! Said with care and concern for you and with as much gentleness as possible, but you absolutely can go NC, you are just choosing not to.