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You may have answered incorrectly during the dialogue with him in the mall, check out this guide, it details all the routes and what to do on them.
https://docs.google.com/document/u/0/d/1LkXWll0_8kVL23hHWWySoRwKL_4dyluPHeLQsnhvwuE/pub?pli=1#h.fkapub2zoqv4

Hey, if you've already met Tristan in the Mall, then you need to look for him in the "Beach" location, and then in the "Downtown" location, in the latter there will be a dialogue in which you need to be friendly with him.