1) If your son received medical treatment, then he has to pay the bills. (Hopefully, he has health insurance.) Anyone receiving medical treatment is obligated on the bills.2) The above said, if your son believes someone else was at fault in causing him injury, he may sue them.3) Your son doesn't have to take the declination of his claim(s) as the final word; again, he can sue, which is where someone has the chance to prove that another was at fault and is liable, or legally resonsible, for his injuries and costs.4) It is hard to see why the airline would be responsible; they did not cause his injury and are not responsible for the hotel's  safety, etc.--not even if they booked passengers into it. The only exception might be the very rare case of the airline knowingly booking people into a hotel with known health or safety issues; that might then give rise to a claim.5) On the other hand, since hotel rooms should not have excessive carbon monoxide levels, the hotel may very well be responsible, and your son should consult with a personal injury attorney on the matter.