"When I was a little kid, I started with B. Then it was Ber and then Bern and now it's Bernard. Or the Bernster. I've never called him Bernie. And I never have called him Dad," says Levi. "Even when I was six years old, I thought it was childish. He was a friend, not an authoritarian."
This is just sad. What his son calls him doesn't matter, whatever they like and is comfortable for them is fine. What's sad about it is that Levi thinks that Dad = authoritarian. "Dad" isn't a bad word.
There are all types of dads - some good, some bad, a lot in between. I can see wanting to reject a bad one (which no one is saying that Sanders is), but rejecting the word "dad" just seems to be rejecting the whole concept of dadness.
By contrast, the Trump kids usually refer to Donald Trump as "dad".