DLA to PIP

My 21 yr old son has received his PIP2 form which I will complete on his behalf. with his input. He has had complex mental health issues since the age of 14, emetophobia, panic disorder, OCD, social anxiety and agorophobia. He does not leave the house at all and has had no support from the mental health service for the last 2 yrs as they say he will not engage with them. What they actually mean is he cannot attend appointments outside of home and they do not have funding for home visits. His only medical support is our GP who just keeps issuing repeat prescriptions. The GP has only met him a few times and made it very obvious he did not like doing home visits. I am concerned that because there is no recent medical input from the mental health service how do i provide evidence to support his claim. Should I send the original diagnosis from when he was 14 and also from 2 yrs ago to show nothing has changed or will there be no point doing this. Any advise would be appreciated.