As I understand it you have the right to request reasonable adjustments to accommodate your condition. This could include adjustments to your workstation , for example. A risk assessment is a natural part of that process.
Here's thought. Why not come at this from a different angle? If there's an adaptation you need such as a special mouse or keyboard, perhaps even a more powerful PC so you can use dictation software instead of typing (assuming yours is a desk job) perhaps you could ask for it and see what happens?
But if you are comfortable at work and don't need any of that then perhaps your boss is right, you may not need one? Let's face it, you have enough to contend with without drawing your employer's attention to it.
Paul Barrett
Hexham - Northumberland - Loads of spectacular walks - all I need now are the joints to go with them! :)
Enthesitis (2012)
Ulcerative Colitis (1990)