Unless you sold him the land, his claim be challenged, and you have until the memory of man runneth not to the contrary, which in most American states is 20 years (but some are less). You may be unable to use the small claims court since many states would require this action to be brought in a court of general jurisdiction since it is an action to determine ownership of land. If you are in the US, you need to check your deed (in some countries you’ll need to check an official registry). In any event you need a lawyer. Disputes over land are hardly unheard of, in part since even when buying a house many purchasers don’t have the property surveyed. If you have title insurance, you might also file a claim since if your deed included the land, they are required to defend your claim.
P.S. Your reference to a “land office” suggests you are not in US, since a central land office sounds like a Torrens system – so contact a lawyer in your jurisdiction. Anyone who handles real estate matters should be competent to deal with this.