I've been guilty of visiting him myself, even considered taking him home to my girls since I don't live in the city limits.
Most likely he decided to have himself an adventure and wandered off on someone. Old English birds have a very healthy wanderlust. In fact, if you haunt the BYC forums long enough, you'll see that almost everyone who posts "gosh I found this bird in my yard/tree, what is it?" almost always has an OE.
You can catch him and see if some homebody hens will outweigh his sense of adventure, but even that might not work. Good luck!