UPDATE ..
I have just read this section several times
"one parent is living and settled in the UK or is applying for settlement at the same time as you, and has had sole responsibility for looking after you"
I am sure this means my wife would also need to PROVE that she alone has been the one looking after him along with obtaining the fathers consent ...so she would NOT need a legal document stating that she has sole responsibility (wich could only be obtained by going through the courts) she would just need to prove that the son has always lived with her and she has been the one supporting him..