It may very well affect his benefits, PIP and therefore his motability.
I do know the DWP must be informed if you have a stay in hospital, even for a day l as it does affect your benefits , I would guess being in a carehome is similar situation.
But you must take advice from a benefits advisor, or Citizens Advice. Try your local Maggies centre.
We have a really good advisor at our local Maggies Centre and knows the system inside out.