Not sure exactly what you asked him to write, but if he is reluctant to write something confrontational to your employer, why not ask if he can simply put in writing the strategies and adaptations you need to function effectively in the workplace? Explain to him that your workplace doesn't have an occupational health department, so he is the nearest thing you have and you need a return to work plan which takes account of your information processing difficulties?

I don't see how he could reasonably object to that, since it's his job to test and make recommendations regarding your cognitive difficulties. At least then you'd have something to show your employer and attorney regarding reasonable adjustments they could make, signed by a health professional. He wouldn't even have to address it to your employer, he could address it to you, like he was just providing you with the information for your own use.

If you have an email or postal address for him it might be better to write and ask him for this than to tell him in person or on the phone. That way you can make sure you've worded it in a way that will make it very hard for him to reasonably refuse, rather than risking getting overloaded trying to put it into words and risk him not understanding or being able to say no more easily.

I am doing a research project into PCS at the moment. One of the findings I am coming out with is that people with this condition desperately want health professionals to help explain their needs to family, friends and employers, but health professionals tend to see it as purely a problem with the individual and will not involve themselves with this sort of thing. It is very unhelpful. I think until this changes the only way forward is to take charge and tell the health professionals exactly what you need in writing.