A lot of us probably have our theories but I think there are lots of possibilities here, since there are still lots of unknowns. And of course sometimes the most likely scenarios turn out to not be correct. Plenty of possible overlap too, where more than one thing could have happened. Did I leave any out?
James: left by his own free will, was lured/tricked. Was kidnapped or coerced.
By/with/to: an older peer, a misguided adult, a peer with a misguided parent/guardian, a family member, a predator, a trafficker.
Goal: To escape something, seeking adventure, missing someone, thinks he is in love. Or involuntary/coerced.
Or James is no longer with us: suicide, murder, accident.
The mother being out of the picture for four years actually seems imo to add a little hope, considering some of the other possibilities. I believe he also has a few adult siblings. Perhaps grandparents he's been out of touch with for a few years, as well? So who knows...
Also, bonus question, if anyone is up for it (and to clarify, we've seen no evidence for it in this case, so it's purely theoretical): If you had a young teenaged son, brother or grandson, and believed they were in a bad situation (whether true in reality or not) would you take them in, even if it was against the law?