Keep in mind that the onboard O2 is for use in emergencies.
The O2 is not just for medical emergencies, but inflight emergencies such as depressurisation - allowing the crew to attend to passengers following such and event.
It really is no different to travelling with medication, insurance, emergency funds etc - if you can't afford the basics to keep yourself safe and healthy, you really can't afford to travel.
As for the airline charging for the extra seat - well it sounds a tad greedy on their part, but it does of course depend on how much oxy he will need etc...