We wanted the mobility issue mentioned (very obvious when you see H that his mobility isn't good)
It would make answering your question a LOT easier if you said what you wanted the evidence for.
Does he have a Blue Badge?
If not, apply to the Council you are under and ask for an assessment by their staff, they will assess him for you and you will have a record of how far he can walk and a fair few other mobility related things on their assessment form which you can request a copy of.
Some Councils will no longer automatically renew badges unless the badge holder receives Enhanced Rate PIP Mobility.
If you're going for PIP, then you'll be seen at a Face to Face Assessment and his mobility will be assessed there by CAPITA or ATOS. You MUST request a copy of that report as you will need that if you need to appeal because it's not accurate.
I'm not going to write more at this point as I could be wasting my time advising for something you're not doing, apart from saying if PIP is what you're going for, use the online CAB guides or join Benefits and Work for their guides and if you have FB, find your local PIP support group. It can be a very difficult system to navigate and if you are refused, appeal, appeal and appeal. Over 70% of NO decisions by DWP are turned into YES decisions at Tribunal, even with the same evidence.