Maybe he felt bad since it sounds like Alan and Bryer discussed it (maybe even with Kam included) and it was sorta decided they would work for a few months and then do the whole “leave town and find work”. For some reason they decided to leave early (maybe Kam was anxious to get going?) and Bryer agreed to it. Not wanting to disappoint his father he was reluctant to tell him and waited until the last moment.
FWIW, probably nobody heading to Whitehorse in a "few months" because that would be November. Locals have told us that they should have gone in spring to get a summer job... so they probably should have waited until 2020 if they were actually looking for work. It did sound as if AS expected BS to keep his Walmart job longer and to save $.