So, why do you believe you are right and he is wrong? The vulnerable become more aware of their surroundings and how it affects them. They can see their physical, mental and emotional reactions to stimuli. Part of it is an impatience with the mundane, their lives have been reduced and simplified to what is important, necessary, and useful, the rest becomes garbage. Trivial becomes irrelevant. It is a new vision about how people waste time, taking for granted tools and skills that may be gone tomorrow. Nothing is now ever taken for granted. It becomes a new land of black and white while living in a land of grays.
Adjusting to a diagnosis of ms is a proccess, not an overnight change. The mind needs to sort through new facts of physical and mental aptitude while adjusting to the quirks of ms. Like learning how to walk, the child sometimes gets angry about not achieving the goal immediately, but time and practice makes it a certainity, not just wanting it.
Besides the many losses he is grieving for, there are the new physical changes that must be identified, acknowledged and accepted.
Think about giving him a wide berth and let him work it out for himself.