Performance Measurement Visit

My son gets the higher rate of P.I.P for autism and other related conditions, he was awarded in 2015 until December 29th 2019 with review date from Dec 2018. I have been his appointee since he turned 16, he was 19 10 days ago.
This morning I received quite a threatening letter titled Notification of Performance Measurement Visit for Tuesday Morning. I have no idea what this is/ It states that it will take 1 1/2 to 2 hours to complete and says a few times about making sure all information is complete and correct. if it isn't and we have paid too much money you may have to pay the money back . You also risk being prosecuted or having to pay a financial penalty. It then goes onto say After the review if we have paid too much money because you have failed to tell us straight away about a change in your circumstances, or you have provided incorrect information, you may have to pay the money back. Your P.I.P may also have to be stopped. You may have to pay a financial penalty or face prosecution which means you could get a criminal record.
It has three errors throughout with different peoples names on asking for I.D. and prescriptions and any professionals details he's in contact with. The only change in circumstances was last year he went from college to university, he still lives at home and is monitored from a distance at uni. I take him and wait outside then he comes back to the car after lectures and i bring him back home in between, but he is still in a structured education routine.
I suffer with social anxiety myself and don't use a phone so i can't call them. I feel like i am being called a liar and the letter is threatening. Does anyone have any idea what this is about please?

Hi thanks for you reply Gordon, just an update and a couple of questions if anyone could advise please.
Had the interview today, the interviewer was personable and friendly, he wanted to see mine and my sons I.D. then explained he was there to review the claim of P.I.P. after a brief chat he started to ask the questions that are on a P.I.P form, saying he had read a copy of the 2015 claim form.
I then asked was it ok for me to refer to my notes to which he agreed. I explained about UNI and the support in place to allow him to do the course etc:
The interview (assessment) lasted for 45 minutes, he did not ask to meet my son or look at bank statements etc. His phone rang 3 times during the visit and he said he had made brief notes and would fill the rest in later when he had time. He then asked me to sign a form and a letter which he explained was what we had discussed and also authorisation to contact my G.P.
On the way out he said that i would receive a letter in 3 to 4 weeks and everything will stay the same as he is already on higher rates of both, he then said some people don't even get near 12 points and not to worry, the condition is also unlikely to ever change.
So I was so tired after being up all night with a very distressed autistic teenager and signed the form without reading it, something i have never done before.
My question is, what is likely to happen next?
would i (my son) be called for a medical on the basis of what i discussed or would i have the opportunity to fill in another form first?
Will i get a copy of what i have signed?
Can i appeal if he is not being completely honest and the score goes below 12? he scored 31 on care and 12 on mobility in 2015.
I'm ruminating now and worrying as we all do.
Thanks in advance for any advise given.

Dear Ejay

I too have an autistic son who has just finished uni. It took him 5 years to complete a 3 year course due to his difficulties. You don’t say on your post if your son is in receipt of any type of disabled students allowance (dsa). My son had a mentor and autism care support worker to help him with all aspects of uni... The needs assessment report was used as evidence when I did his application for PIP. Your son probably already had a uni needs assessment but thought I’d mention it to you just in case he hasn’t

Flora

Hi Gordon, thank you for your reply, i was just wondering what options will the decision maker have please? is the form he filled in now to become the pip form for a fresh claim and if so will i get a copy or can i request one? it's never happened to us before and i have no idea what will come next. Could it be revised with a new date for renewal as it was due to be reviewed in the new year? I am pleased i will be able to appeal. I am surprised they can do this when the investigator is not a "HCP"