the old fence is the boundary line on the deeds and it states both parties are responsible for the upkeep, yes he can build a wall on his side but he is still responsible for part upkeep on the boundary fence, also he needs planning permission to build a wall higher than 2 meters, and a wall which he has built 3- meters only being 3 inches thick is against building regulations, which is laid down the thicknes they must be for different heights, i had this out when i built my garrage which was put up by a firm its a concrete one, they kept it back 3 feet of the boundary into my land, this is so the wall belongs to me, if it had been put up on the boundary,half would have belonged to my neighbour,