No, he is not an abstentionist. Here is his position as stated in the sane podcast:
“And if you’re one of those progressively minded people that’s convinced themselves, half rightly, that it doesn’t matter who you vote for, it doesn’t matter if you participate in the system, it doesn’t matter if you protest, keep in mind that it’s not that cut and dry. The system is a corporate totalitarian system in its very nature, but it can indeed get far worse. And that’s what you have to fight back through whatever in-system mechanisms you can, as we all work, of course, toward the true solution, which is total system change.”
I think that voting will be one of the “in-system mechanisms” he thinks people can use to try to stop things getting worse. I imagine he will call on people to vote against Trump, as he did last time.