I don't think he should run. This year proves that populism works. Sanders was able to build a massive movement, and the entire millennial generation will be able to vote for the first time in 2020. This year, it was no secret that older millennials (the ones who could vote this year) preferred Sanders. Those voters could easily swing to a progressive candidate like Brown, Warren, Feingold, Ellison or Gabbard.
If he did run, I think he'd be swamped.
Millennials will not be taken seriously until they act like mature adults and not like whiners, like some of them are in the streets.