By 2007, mobile phones were exceedingly popular amongst teens...but only if you had a social network/someone to talk to. Their primary function was phone and text. The camera was horrific and there wasn't enough memory to store hoards of photos. The social media apps we know today didn't exist yet. Internet access via phone was also a novelty, very expensive and had a limited function. It is plausible to me that he didn't have a phone.
On the other hand, the Internet was booming - forums like Yahoo Answers, Digital Spy etc were growing. YouTube, Wikipedia, MySpace, MSN Messenger...I could go on and on. It happened quickly, and by 2006/07, broadband was becoming a must-have for households. As a teen with niche interests, who didn't have many close friendships, the Internet gave him access to any subject he was interested in, as well as likeminded people to talk to.
It can only be the case that he either accessed something by himself...a book, a magazine, a website etc (to spark the idea in the first place) or more likely (as this was carefully planned and unusual behaviour) he was in contact with someone, somehow. I think the fact that they found nothing on any devices, is simply a red herring. He was clearly very clever and good at hiding things. Someone had to have been spurring him on...
It really reads to me like a typical way to arrange a day out, back then.
He had been chatting to a person for a while, they do not seem like a threat. The person recommends meeting up for a few hours, to hang out or to go to a specific location. They pre-agree a date and time. Andrew knows (or believes) his parents won't allow him to go, or he simply doesn't want to explain himself - he is 14, after all. He leaves early, as he has to get back home before school ends and his parents got home. He meets the person in a mutually-known location, and it all goes wrong from there...
"Oh no, I forgot xyz, can you come back to my place so I can get it?"
"I'm so and so's dad, he wanted me to pick you up instead, is it ok if I drive you there?"
"Why don't we do xyz instead, it won't take long?"
"This is my friend xyz, he wanted to go to this place instead, is that ok?"