I star from the idea the American establishment did not want him to run for president from the beginning of his first presidential campaign and most of media made fun of him, even Fox; so he is definitely not part of what I would have called the common establishment, otherwise he would have been accepted easily among the ranks of the Republican politicians at the time, he has only continued to be politically relevant because he has forced to party to pay attention to him, by captivating the willing and power of the vote, from the common Republican voters who got radicalized.
He has forced the establishment to co-existing within, for now (the Republican establishment) and the democrat establishment tried to oust him from this race since he started to show actual intentions to seek for a second term. Both establishments hate him, one secretly and one openly and with no shame about it.

These are going to be very weird years under a second administration of Trump, one must wonder who much he will go out his way to try to hurt the establishment as we know it, specially the Republican establishment.