I doubt there's any one certain way to keep a kid from losing interest in baseball. Some will and some won't. If your son has been playing that long with enthusiasm.......that's awesome!
You mentioned the thing about winning. I don't think I have a "for sure" opinion on that. I've heard lots of parents and coaches say keeping score is bad, winners and losers are wrong. That's probably true for some. But I'm not sure it's right to "quench" the competitive fire that most boys seem to have from a very young age.
Billy Martin made a pretty good point about winning. Go to youtube and check out Billy Martin on winning.
CWM