Hi and Welcome to the forum, you might want to have a look at the following FAQ which explain where everything is
Welcome to Benefits and Work Our PIP guides clearly explain how to describe your son's limitations in such a way that he has the best chance of scoring points in the relevant descriptors. I'm afraid you cannot make the descriptors fit the facts, you have to work the other way round!
PIP help for claimants You might want to bookmark/favourite this on your web browser now so that you can find it easily in future. That will allow you to return with further questions/comments without having to start a new topic each time. It helps us enormously when everything relating to the same claim is kept in one place - and I hope you will find it useful too