This is odd on and infuriating on so many levels. Not only was he ok doing this to a little child, he was ok doing so in a car full of people and he was ok telling you about it. Why would he think you'd be someone he could tell? This tells me that this person hasn't got a normal conscience or sense of shame. Hopefully he was exaggerating or hopefully for the children he's encountered since then, this was an isolated incident, but I doubt it was.
I find it disturbing that given your lack of action, you were even able to relate this story publicly. Even at 17, (especially as a JW) I'd have been compelled to tell someone what he did. Who knows what irreparable damage he's done to other little ones since then. I can be understanding about this to a point but mostly, I don't know why you'd bring this up if you were going to make excuses for him or weren't going to do anything about it. I'm not sure if you really comprehend what a big of a deal this is in terms of what his history might have been, his position as a JW, what he may have covered up or have done to other children himself.
I like Simon's idea, find him and tell him what you remember. If it were me, I'd tell him that the JW child abuse scandal has brought this memory back to you and you aren't sure where to go with it . Tell him you want to get further clarification on the matter and want to know what's going on with him. At the very least, if he has in fact continued on this way, he'd get the message that someone knows and is watching.
If I found out anyone did something like this to my child or one of my little granddaughters, I'd end up in prison for manslaughter.