Sell the man a damn seat with a ticket stating it has a width of 17 inches and that is that!
If the man obstructs the evacuation, or egress of other passengers, then it is a Health and Safety issue and he may be declined his 17 inches of paradise!
Non obstruction of egress of others needs to be a condition of carriage.
As an airport caterer I entered the back galley of a 733 once at WLG. The bulk of passengers left the plane, but it took 24 minutes to get a quadraplegic from the aircraft on a 30 minute turnaround.
Tickets need to be written such that any person who obstructs or potentially the safe evacuation of others may be declined carriage.
Then you put the problem back on the obese person and then you offer a solution of another seat.
Also why not sell tickets which state that the sale of each ticket entitles the bearer to a maximum agregate weight ?
Then if some individual wants to exceed that by more than 10% irrespective of their personal weight, they are obliged to purchase a second ticket ?