Hello!
It is a pain in the arse to write a lot about what your kid can't do, followed by why you/he can't 'just' do an 'obvious' thing that would fix it.
However, if you have a diagnosis and follow the formula text for each question, it should be fairly straightforward to get the points for standard rate care and possibly mobility. Probably for three years and then when they send you a review doc, you put unchanged and can add a couple anecdotes of problems.
My kids are younger so have DLA for ASD, but I've done a few PIP claims for me and others.
Key is to start well in advance gathering reports, records of prescriptions, etc, and drafting text for the question. I would type all answers in a Word doc, so you have a saved copy, can amend in the future, and also it makes it easier for assessors to read. You usually get about 3 weeks to fill in the PIP2 form, as it tends to take forever via the special DWP postal service, so best to be ready rather than trying to phone for an extension.
It is worth paying 16 quid for the Benefits and Work guide, as they have guidance on how to answer questions for common conditions, and also include the guidance assessors have to work to in deciding points. Bear in mind that it's all about 'how your conditions affect you', so if you say he never goes out to socialise, the reaction is 'fine'. If you say he has given up trying to socialise BECAUSE of the reactions, BUT if he had some money for a paid buddy he could practise going out and trying x activity... that gets points.
NAS have good guidance, FB groups often do too. Good luck.
And send the claim via signed-for delivery - it's amazing how many get lost until you point out it was signed for at their end and magically found...