PIP review

Hi,
My son's PIP was due for review in November of this year but we got the review form in January. I understand it's not unusual for the DWP to send it out a long time in advance like this.
I submitted the form on time but have heard nothing since. My son's health has deteriorated since I sent in the form; he has autism and an anxiety disorder, plus ME/CFS. His anxiety disorder has became a lot worse; It has meant for instance he has been been unable to continue with a course of therapy he had started for for the ME/ CFS (I had told the DWP he was due to be starting this). It also means he would be unable to travel independently (he only receives the higher care component at the moment, no mobility). He is on a new daily medication to try and control the anxiety.
The review was by paper form, no online option. Should I contact the DWP to inform them of the change and if so how-write a letter to the address to which I sent the form? If so, in what format?
Or should I wait to tell them this when they do an assessment, which is presumably what they are planning to do?
Thanks in advance!

Many thanks Gordon, that's very helpful. I will get that in asap then.

I called first, to tell them that I was writing, just in case they had already started looking at my case.