According to the Secretary of State's website, the filing deadline for this office is August 8. Thus, Sanders still has plenty of time to officially throw his hat into the ring again, and under current conditions, he would be the clear favorite to win again regardless of when he officially files to run.
If Sanders were to bow out so late, who would launch a winning campaign so late? For sure it's much easier in a small state like VT, but kicking off a senate bid in June or July, let alone early August, is kind of stunning.
I think Becca Balint would be the natural choice to jump up in if that were the case, but I doubt Sanders doesn't run for re-election.