The passenger may or may not have looked like he was a jackass, but I would encourage his to get a lawyer.
According to United's Contract of Carriage, it appears the good doctor was in the right, and United was wrong. United's Contract of Carriage speaks only to DENYING BOARDING to confirmed passengers on flights that are OVERSOLD.
https://www.united.com/web/en-US/content/contract-of-carriage.aspx#sec25
United didn't try to deny the his boarding, they permitted him to board the flight and then physically removed him from the flight. Second, the flight did not appear to be OVERSOLD, in that United did not sell tickets to the individuals it attempted to give preference to over its paying, boarded passengers.
As such, Asian bro was an authorized licensee of United and his presence on the plane was lawful at all times. United attempted to unilaterally and unlawfully revoke his licensee status, and he was within his rights to refuse.
EDIT: The above is based upon Rule 25 in United's Contract of Carriage. Rule 21, which lists the circumstances where United may refuse to transport a ticket holding passenger, further supports this interpretation of the Contract of Carriage.
His situation was clearly a "refusal to transport" by United. However, overselling the flight is not listed as a reason in Rule 21 for United to refuse transport of a passengers. The only relief afforded to United for oversold flights is in Rule 25, which is limited to "denying boarding." The Contract of Carriage's Rule specifically pertaining to situations where United may remove a boarded passenger from a flight dispels any remaining doubt that Rule 25 concerning "denied boarding" can be interpreted to apply to forced deboarding.