Now for my confusion, was the boy with the Shiny Dustox the boy from the flashback or just a look alike, cause if it was the same guy wouldn't he have aged, at all?
As far as I can see, the guy would've at least gained some height. But concerning your question - that was never resolved, if I understood correctly.

An argument against this would be that even after the Dustox guy told Jessie his name, all she had to comment on was that he looked similar to someone she knew, which suggests that the boy from her schooldays has a different name - otherwise (in my opinion) she would have had a stronger reaction and actually considered that they were the same person, which she didn't.

Also, what was the plot of the whole flashback, cause the only one I saw was the one where he gets on a bus, then Jessie chases after him, then trips, it didn't make much sense to me.
Jessie and the boy were schoolfellows (at least I think so, judging by the uniform), and she liked him. He was about to start a journey to become a Top Coordinator and actually asked Jessie to go with him, but she declined, saying she would attend a local audition with two other friends and try to become a singer.

Jessie ended up failing at the audition while her friends could both advance to the next round, and regretted her decision not to follow the boy.