It may be a longshot, but have you checked everywhere in the area he roams for places he could be trapped? Hubby's former neighbor's cat disappeared for a week and was found in a neighbor's tiny shed (attached to the house) where they had the water heater. Hooligan had snuck in when they had opened it to relight the pilot light, they momentarily left it unattended (and Hooli got in to investigate), and they shut the door, not realizing he was there. Luckily after being trapped in there for a week they found him and he recovered from the ordeal.
Also, if he's injured, he may be hiding in some small cranny or hole nearby.
I hope you find him.