Props to you for helping out as a friend. A few random thoughts in case any of them are helpful:
Getting him to the best hospital possible, wherever that is, sounds like the best first move.
Reddit is a great idea - the breadth of audience there is amazing. There's almost certainly already some subreddits that deal with helping diagnose unusual symptoms.
Not trying to find a match for the symptoms necessarily, try googling weird diseases and how they were diagnosed... every now and then I'll read a story about someone that had strange symptoms and it took them a long time to finally get it diagnosed. Maybe reading one of those cases will give some more ideas on a way to get help, or provide something unexpected to consider.
Maybe try to find a Hawaiian doctor/resource to help out?... they might be more familiar with something local that causes some of those symptoms. Maybe focussing on a few of the symptoms will help lead to an answer if the son turns out to have an unusual combo of symptoms that are complicating being able to zero in on the primary symptom(s).
Ask the family write down everything they remember doing while they were in Hawaii, maybe that will help jog their memory (or give a doctor a clue) of something they did which could provide an area of focus. E.g. did he go swimming in a pond?... walking in an area where there might have been insecticides, pesticides, mushroom spores, ???
If you can, please keep this updated when there's news, I really feel for the kid and his family.