PIP rejection 16 yr old - what next?

Dear All 

My son has just turned 16, he was awarded DLA till he was 18, with ASD, hypermobility, and skull defect. Then they wrote to me to let me know they were moving him onto PIP.  I followed the process and was appointed his appointee. Sent in a letter, and all the other medical requirements, Attended the interview and explained how we have to do many things a certain way. The assessor was really pleasant but the write up that we have received bears no relation to the interview or the questions he asked. It says things like you did not appear anxious, you had good cognitive understanding, your mum cooks but there is no suggestion you cannot learn. You have a bank account and one day you will learn to use it, this kind of thing. This was after we explained my son doesn't eat if I am not in the house as he is too scared to cook as he left the cooker on all day once. I wont go into all the details, as I am sure you are all aware of the normal problems, severely disturbed executive function doesn't seem to exist according to this write up despite the consultants and psychologist explaining it does.

I also noticed the letter was apparently written three weeks ago, yet just arrived yesterday which I find very odd, leaving me only a few days to work out what to do. Do I bother writing back or is it pointless? Many thanks