Not a lawyer, but this should hinge on contract law, as well as the ticket's fine print. You're absolutely correct that if this had been handled pre-board there would have been no incident - nasty crap happens all the time in airports. But - if they issued him a boarding pass with seat assignment, then allowed him to board? There must/should be a point at which the airline commits to providing the passenger with transportation as a contract commitment and can't renege.
And maybe not, but can you imagine getting a persuasive lawyer in front of a jury full of pissed-off airline customers (that part shouldn't be hard)? Unless something else than what we've heard so far pops up, this guy is in line for a big payday.