If he didn't like to be alone he wud move on. Maybe he has just had it with a bunch of nagging old Biddies always nattering at him and decided "enuff already. I'm going to live alone"If you throw him some feed and maybe a water dish he will probably stick around.

it varies from breed to breed and feed and living conditions anywhere from 5 years to a few up to 20 years though the average is about 10 years

your rooster was probably ran off from his flock by a more dominent rooster. Usually one rooster will rule the roost and make life hard for any others.

just an after thought. if you did get a mate for him it should be at least 6 months old. and she would probably start laying eggs soon afterward. so unless you made a chicken pen with protection for them from predaters or dogs and cats she would build a nest in the thick grass or around a shrub. she would not likely roost in the trees but would sit on her nest once she started laying. most hens lay 200 or more eggs a year for usually 3 to 5 years but a few of the heritage breeds lay for 10 years or more but just not as many eggs. So the next step would be to get 2 or 3 or 4 females so you would get enough fresh eggs to eat or cook with. or stick with the one hen and let her hatch baby chicks in a month or so. but then you would need a pen or enclosure for sure them so the chicks would be protected at night and when the were running loose. Some towns have laws about livestock in city limits and others don't. also skunks or raccoons or snakes feral cats like to prey on the chicks. Or even eat the eggs.