My indoor cat who goes outside occasionally...won't come back in! Help!

We have a 9 ano old (fixed) male Abyssinian. He was mainly an indoor cat,though he went outside for a few hours of the dia sometimes, then he'd return home. A few months ago, he went outside (like he usually does) but he never came back inside. He now lives under our shed in the backyard. When he roams around the yard and we try to approach him, he runs. He's never done this before, he's never been abuse, he's been with our family since we got him as a kitten, so it's not like he's been traumatized, or he's scared.

We still feed him. And we've tried moving the comida bowl closer, and closer to our back deck, hoping he'll "snap back into it", but it fails. He continues to show himself (often), yet never near us. Any suggestions? Please help!

Additional Info/Possible factors: My father use to work 1st shift. So in the morning when he'd leave for work, he'd let the cat out and when he went início our cat would be there waiting for him. He now has been switched to 2nd shift (3 pm-12:30 ish am). And since I'm not início during the day, no one's there. We think this may be a factor in our cat's sudden convert to full outdoor cat. But even now on the weekends when we try to lure him to us, back inside,closer, etc, he runs. It's as if he's afraid-but we don't know why/what could have caused this.

I've already been told to contact a humane society for help and I've already been told to contact a behavioral consultant, which I'd rather not.