If there is no "restrictive endorsement", PLEASE write "accepted as partial payment for security deposit". This way, you're on FIRM ground. That's how I was trained.

As to the PO box, I believe you'll need to check this point out down the local courthouse, though my understanding is you'll need to serve him at his home (if that's where he runs his rental business).

As to obtaining his home address, the USPS requires all PO box holders to have on file their home addresses, though you might need to file an information subpeoana to get it. Check this point out at the local post office and court house too.

The other way is owners of records are public information, on file at the local tax office, or the internet, and you can probably get hold of his address that way.