I am am a wheelchair bound Veteran with multiple health problems including a liver transplant, diabetes and heart disease and I fly with a assistant all the time. A 13 hour flight to London should not be a problem. He isnt hooked up to any machines and even if he had a IV drip on him, which he does not, would not be a problem. Yes, you can fly with an IV drip, but it requires careful planning and approval from the airline. Most airlines allow it if medically necessary, but passengers must provide documentation and meet
So what, why the "veteran" mention? Are you circumstances the same as his? Obviously, you have no idea of that, so, unless you are aware of an airline that has agreed to take him, your comment was not sound.