Hi John
Having the diagnosis obviously helps, and it means that they will take his walking difficulties more seriously. They won't backdate anything—the PIP system isn't built for that.
All you can do now is make a case for a mobility award and make the connections to the diagnosis. Make sure you don't leave room for them to say that it's his choice that he doesn't move rather than the physical difficulties.
Remember that the diagnosis, as important as it is, is only secondary to how the symptoms affect a claimant per the specific PIP criteria.
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems