Road traffic legislation applies to all roads, defined as any highway or other road to which the public has access. It would be for the courts to decide whether any particular unadopted road was one to which the public had access. Where the public does have access to such a road, the police are able to use their full range of powers to enforce road traffic legislation.
so It depends on whether the public has access and seeing how you and your son are members of the public accessing the road I would say the answer is no.