Obtaining GP letter for national concessionary travel pass when getting PIP

As proxy, advocate for my son (now classed as adult) and out of formal education - as he qualified for a free concessionary travel pass whilst in education through the assisted transport, that is not the case now he isn't. We completed the correct form for TfGM and submitted it. However, we have now received a letter stating that we need to provide a supporting letter written bespoke from a medical professional that states in their medical opinion why he would likely be refused a driving licence should he choose to apply for one based on the conditions we described to them.

Is there a template somewhere on this website for me to download and use? Or can someone please provide another pointer for us to go through.

We were only getting a provisional licence for him as photo ID - NEVER for him to learn to drive or actually drive - had to go for a passport instead. We still require the gp letter / template - next GP appointment is 26 July 2023. Looking forward to hearing from you helpful and knowledgable people. Thanks.