I don't know what happened to him either, and I've always been curious. It would be cool if maybe they mentioned him somehow in KOTCS, but I doubt it. For Christ's sake he was practically Indy's son in ToD, so I never knew how they just left that hanging. It wasn't like he was some romantic love interest that Indy had fleetingly. For all we knew from the film, Indy had practically adopted him. Why would he never be mentioned again?
It's bizarre... I never got it?