Mason Disick is no fair-weather fan because he's still wearing Tristan Thompson's No. 13 jersey.

Kourtney Kardashian and her 8-year-old tag teamed it Tuesday in Calabasas both looking sporty.

We're guessing Mason's jersey was a gift from Aunt Khloe, who's been dating Thompson since last Summer. His aunt's bf pick clearly came with perks, including seats to Finals games.

Could be worse ... like still rooting for the Lakers during last year's 26-56 season.