Are you clear about the exact ownership of this piece of land? I guess we are talking England and Wales law?
As I understand it, a leasehold flat has a lease over the land on which all of the flats are built (stating the blindingly obvious!), a freehold flat actually also owns the land on which one flat is built, there may be other leasehold flats on the same site (see above). A Commonhold flat (quite recent legislation - 2005?) owns a share in a company that actually owns the land on which all the flats are built; that company owns the freehold.
The point is, in law, ALL the flats cannot own the land - either one of them owns it (the freehold flat and the others have leases), OR a separate freeholder owns the land and all the flats have leases on it (and pays the freeholder ground rent), OR a commonhold situation may apply.
Regarding the claiming of the land, nice as he may be, this guy can fence this land off and maintain it for > 10 years, thus enabling him to start the process of claiming it from the freeholder by adverse possession (if Registered Land). A piece of paper signed by him, given to you (unless you are the freeholder) may not be enough to prevent this.
More than that, I think you need proper legal advice.
A better solution would be to get your neighbours to assist in doing something better with the land - for everyon'es benefit.