Who would he even sue though? And for what?
I haven't watched the whole video but if he suffered injuries wasn't it at the hands of the police, and not United? Which means he would have to sue the police - who of course have qualified immunity.
Unless United just wants to pay him "hush money" to go away I don't see that anything they did was actionable. If the rules allow them to bump passengers then they are allowed to bump passengers and their liability is limited to paying whatever the statutorily required amount is.
A "police brutality" (Section 1983) claim against the PD is always possible, I suppose, but I don't see United being a part of that.