I don't think he is being held captive. I think he was preyed upon by an opportunist hanging around the station and sadly was most likely dead by the end of the day.
It's also possible that he was abducted off the street by an organised paedophile ring, and he looked slightly younger than 14 so it's not implausible that he was taken and used, but that still means to me that he is long disposed of.
Of course I hope neither of these things happened and Andrew chose to leave for whatever reason, and is alive and well somewhere just living the life he wanted. I do wish that if this is what happened he finds it in his heart to let his parents know he is safe and well. They deserve that peace of mind.