I think PUK have little cards that you can put in a wallet or on a key-ring that say something along the lines that "I have PD and need a little bit more time" ....or something similar. Enough to guide people to be helpful - or pull someone down a peg or two when necessary!
Sorry that your Dad had to put up with such behaviour which is so unnecessary....but so many people are intolerant of others eg. of Learner Drivers, or the parent with an autistic child or as in your Dad's case someone with a hidden disability.
If he had one leg or a white stick or a wheelchair....he would probably have had a different response. What am I saying!! For some people even the obvious difficulty doesn't prompt support and tolerance....I was in my PO on crutches, after breaking my foot, to buy a prepaid envelope he said he couldn't sell me one that day and to return to the PO the next day, I said I couldn't due to my foot etc and he said "Well that is your problem - not mine!" As I left the PO holding back the tears....I heard the customer behind me say "You shouldn't have spoken to her like that," and came outside to check I was OK!
Glad you contacted the head office, they should do something about it. In my case the PO followed up on my complaint and apparently that staff member had a number of complaints against him - of the same kind of behaviour - he is no longer at the same PO!!.........
"Now't so queer as folk"...as My Mum would have said