I think Andrew may have met someone at/because of going to the camp he attended during the summer before he went missing.
Him taking the train alone into London was so out of character, and based on what he took with him, and most importantly what he didn't, it did not appear in that he planned to leave permanently.
His case is so remarkable because of the glaring lack of evidence, especially in our digital age. He didn't have a cell phone, no email, no Internet at home, and no activity on the computers at school. This makes it likely that whatever/whoever enticed him to go to London that day was something or someone he encountered in real life. He had also stopped taking the bus on the way home from school for the few days of the new school year before he went missing, and decided to walk, which could have provided him the opportunity to meet with someone his parents weren't aware of.
I have thought of him and the circumstances of his disappearance countless times. It genuinely has prevented me from sleeping on a few occasions when my mind can't shut off. If the police hadn't fucked up the initial investigation, his family might have the answers they need. Andrew was a sweet, intelligent boy, and he deserves to be found.