My working theory is a nearby neighbor, within close proximity to his own home but out of sight/earshot of it, offered him a ride and things went south. It's possible the encounter started out without nefarious or ill-intentions, but escalated, ending with Jason's disappearance. I think it would have been an older man, familiar with Jason, either retired or with a flexible work schedule. Someone on the periphery--not well known to Jason or his family, but known enough to that Jason would have trusted him upon a closer encounter and accepted a ride, if offered, then be able to slip under the radar. There are a couple of houses close by where he could have been approached without too many people witnessing it due to landscaping and the angle.
To me, this is the simplest explanation compared to clandestine meetings or an unknown lover. From the outside looking in, it appeared random, but this neighbor could have been watching (even listening) to the Jolkowskis for years, saw an opportunity, and made a move.