Wendy
If this is the result of an improvement in his conditions or limitations then you are obligated to inform the DWP of the change, doing so will result in an invitation for him to claim PIP.
I am concerned that you say he has been working for some time and receives Lower Rate Mobility, this is generally awarded for claimants who are unable to leave their home.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems