OK that makes sense. That is not common land - common land is village greens, areas known as Commons - that sort of thing.
The unadopted road might be privately owned or it might be that no-one technically owns it - either way you and your fellow residents do seem to have established rights of access - to pass and repass along the road to get to your house.
However it is a rather different matter if you are wishing to leave a vehicle in the road and thus block another landowner from gaining access to his land.
I cannot say for sure, but I do believe that just because you have been doing it for years means you have gained a permanent right to park there - because what you have an access right not an ownership right.
What exactly are the words written in your land title about this access arrangement?
The best way to solve this would be a little local arrangement that maintains a single entry into his parcel of land - it sounds like he is arguing that the whole of the lane abutting his land is his to control.
You may have to approach CAB to try and get 30 minutes of free qualified legal advice, but otherwise this sounds messy (and potentially expensive legally) to resolve if you cannot do it amicably with simple local agreements.