I think he did go outside to play or walk around, and he came across the wrong person.
I'm glad they are giving lie detector tests to the parents, that should clear them as to any involvement and they can concentrate elsewhere as to what may have happened here.
If this turns out to be some motorist that was just driving down the road and seized the opportunity to snatch up a kid for themselves, there is no telling where he may be by now. If this was a crime, it seems like there was a very, very small window of opportunity for someone to pull it off undetected. How could, or would, anyone plan for a small boy to go walking out of his house after dark while his mom was in the bathtub ?? Everything would have had to line up perfectly in order for someone to snatch him within a few minutes after he went outside. That being said, if there are plenty of street lights around there, he may have been fairly visible to anybody driving (or walking) by, even after dark.
If the parents are cleared by LE via the lie detectors that were mentioned, then I don't see how this couldn't be a stranger abduction by someone who just happened to be in the right place at the exact right moment. People have been known to pull over and snatch up little dogs in the same manner. When they see something they want, they grab it, especially if no one is around.