Hi, my name is AliGimbo!

Hi & welcome 'AliGimbo. Have a look at this website which I hope is helpful:https://www.mentalhealthandmoneyadvice.org/en/welfare-benefits/pip-mental-health-guide/help-with-your-pip-claim/how-to-fill-in-the-pip-form/

The above may indicate the sort of problems your son may face. It's sometimes an idea to keep a diary for about a week noting when these arise. You should then complete the claim form giving a couple of recent, detailed examples as to the difficulty he faces for each applicable descriptor, i.e. when did it happen, where, what happened, did anyone see this, & were there any consequences to attempting/doing any activity?
Say if he can do each applicable activity 'reliably,' i.e. safely, to an acceptable standard, repeat as often as one would reasonably expect, or if it takes him much longer than someone without a disability. 

Keep a copy of everything 'just in case,' & get a free Certificate of Posting from your Post Office when sending the form off.