Hi Nikkijay
If you go through the Guide to PIP claims and reviews - there are sample answers for every single question
benefitsandwork.co.uk/guides-for-claimants/pip. That might help.
I don't know whether you are filling in the form for him, in which case, use "X (your son) is unable to do x x x. If he wants to fill it in with your assistance, use "I ..." Just be aware that the DWP and the assessment services are notoriously bad at understanding ASD - so you will have to spell it out to them.
Don't try and sugarcoat his difficulties. I know that can be extremely difficult if the claimant is embarrassed or unaware of how it impacts on their daily lives. You can always include a letter with the form, saying what you have observed in terms of the PIP criteria and what impact his difficulties have on himself and any friends and family.
If you have any further questions, come back and ask, and we will try and answer them.
BIS