Hi Kevin
You can say that you have investigated the possibility of physiotherapy to possibly improve your nephew's condition, but he can't have it because of the dangers. (The physiotherapist's letter can confirm the exact current state and any other medical evidence you have). Quote from the letters in the course of writing the evidence as well as attaching the evidence at the end.
Can't really comment on the content of the letter - only to say that if the physiotherapist thinks the condition if life-long it would be helpful for them to say so,.
Nothing on this board constitutes legal advice - always consult a professional about specific problems