Ginger
I assume that he has no restriction in his walking distances otherwise.
I think you will struggle to show that he is restricted, your only route is to show that he cannot walk safely, you will need to show that the likelihood of his falling is significant and the risk of injury is high. There's a section in our PIP Claim guide that covers completing the PIP activities safely.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems 
