Bus pass is local authority issued and NOTHING to do with DLA, so he can have a car on DLA mobility AND a free bus pass.
Also despite what people think you do NOT need to be on HIGH RATE mobility to get a bus pass, so long as your doc signs a form stating you suffer severe pain upon walking is enough.
Though this and a few extra tiny bits is pretty much the qualifying conditions for dla mobility, he should also apply to local authority for blue badge, if not already.
tabetha