This is my first post, but I've been a lurker on Websleuths for awhile. I've also been aware of this case for a few years too. Forgive me if these things have been said or I present this poorly or non-SOP!
Here are a couple of my thoughts for random theories:
I think the Anabaptist angle has some plausibility, but it doesn't necessarily have to be Amish/Hutterite/Mennonite, etc. He could have come from an Evangelical or Southern Baptist or Jewish upbringing (or a number of religious backgrounds) that are less accepting of certain lifestyles. Maybe he got into drugs for awhile or just experimented and his deeply southern baptist family disapproved and disowned him; maybe he left of his own accord because of their strict rules and they didn't think he was necessarily "missing", just transient/living a new life. The estimated age of 16-21 puts him at an average of 18 and a half, which is legally an adult, and the perfect age of someone who left home or was kicked out legally (no CPS/LE/school officials to notice his absence) and has been on his own for long enough to know how to get around hitchhiking, but not to have lost markers of his middle class upbringing (good dental hygiene, etc.).
If it is true that a sister called the ME with specific non-public descriptions of him, then it's possible someone from his family heard about the Doe on the news and thought it might be him, but the rest of the family did not approve of them looking for him. Maybe she was too young (a minor) to collect the body, much less have the resources for a proper burial.
He might even have been gay or had some other reason that a family of certain beliefs disapproved of him and wasn't looking for him (if it wasn't drugs, since his tox results came back clean). I realize the note from the Carolines looked somewhat flirtatious, so that would rule out being gay, but I think I read on here somewhere that the letter was found outside of the vehicle and not in his pocket as originally thought, and LE said it was a dead end.
I also do not think that he was a deadhead in the typical sense. He might have loved the band as much as any other deadhead, but I don't think he was following the tour and was in with that crowd as a regular. I have never been into the GD, but as with most types of die hard fans, it is pretty unusual to buy a T-shirt from a tour/concert/show and wear it either to the band's concert or immediately after. With non pop music it is usually unfashionable to even buy the "corporate" shirt. If he was from a stricter upbringing, it would make sense that he didn't know those scene faux pas or didn't care because he wasn't part of the crowd. I also think if he was a deadhead, he's have gone North to the next show, which I believe was in PA?
I definitely don't think HIM was popular here at that time (in reference to the possible heartagram or star tattoo), or even known really. He'd have to have really known about them very, very early, which IMO means he would have to have known about them from being in Finland at some point, which seems implausible. This was also not the age of huge internet use of social media. Livejournal would have been an early social media avenue, but I don't think that was even launched for a few years yet. And not to say that people can't be fans of multiple genres of music, but HIM and GD are not too similar. Based on those facts I don't think the tattoo was from HIM. Probably a poorly homemade star. Stars have always been popular tattoos (homemade or professional).
I think the foster kid aged out scenario is also possible. He would have either entered the system later in childhood/teen years or had a "good" group home/foster parent situation to keep up with his dental health.
