Unless he was intoxicated, belligerent, or tried to assault the security officers, he probably has a good chance to collect damages.
I read he re-boarded after being removed and explaining his situation. If true that makes this worse.
He was knocked out cold and bleeding - when he came to he was delirious and just walked back on the plane going the isle yelling “I’ve got to get home! I’ve got to get home!” and bleeding all over the place and they had to clean the plane. He was escorted back off after this was discovered.
Says a lot about security that he was apparently able to just casually get back on the plane in his stupor...not to mention the fact he was not taken for medical treatment immediately.
The next United flight available was apparently almost 24 hours later. That is not an accommodation to be put on another flight.