This is an old thread but why not bring it back up again?
I feel like this case probably has answers close to "home." (Not necessarily meaning family members, but at least people close to him.) I feel like both he and his dogs were killed by someone he knew. Perhaps he had plans to meet someone that his family didn't know about, or maybe he just ran into someone along the way. I would have started with the people closest to him: neighbors, coaches, preacher, teachers, etc. We're talking 45 years ago, too, when things were handled much differently. If I were investigating today, I'd look at his behavior preceding the weeks leading up to his disappearances. Had he become more withdrawn? Was he dressing differently? Acting secretive? Were there any signs of abuse? Maybe I've seen too many movies but my first reaction was that he was a good-looking, younger-looking, kid. Was someone in his life taking advantage of him and did they panic when they thought he might tell?
My second theory would be that he and the dogs fell into a well or some kind of hole, especially since all three are missing. Seems like those would've been looked into, though.
I don't think this was a stranger abduction. Most abductors like to take action as quickly as possible. I find it hard to believe that they'd snatch a kid with two dogs-it just would've taken too long to get them all in the car and then they'd have had to deal with killing three things instead of of one.