I've just entered 12 points. (I then clicked the option not to save my details as I progress, which bypassed the need to enter my email address and date of birth, but I can't see that it will matter anyway). I then got to a page that simply asked me to provide the relevant details and to upload the letter from the DWP. (Screenshot here:
https://ibb.co/ByG21ZT ).
So it appears that there's absolutely no need to provide any information about your son's condition. The DWP letter, indicating a 12 point score, is all that's needed.