The trouble with these situations is determining where the boundary is. Under England & Wales law, virtually all boundaries between land are defined by what is known as 'general boundaries' as distinct from 'fixed boundaries'. This means that boundaries are shown on LR title plans without referencing distances or fixed features on the ground.
The real questions to ask yourself is 'why is he doing this now' and 'what is the scale of his claim'. Usually these issues crop up at the time of construction.
In answer to the first question, only you can take a view - has he fallen out with you, is he after money, what does he really want, what does he think the remedy is.
The second question relates to the scale of the claim - is he saying the wall is actually built over (his preception of) the line - or is it just the guttering (say).
Above all, you want to try and resolve this without recourse to solicitors who make a fortune from such cases. And if the solicitors costs rack up, both sides start becoming entrenched in their positions and won't back down, meaning the costs rack up even quicker. Someone has to lose.
Try discussing it with him patiently and calmly to see what he wants. Take photos for your own benefit of the current layout of the features. If you have semi-house with him, it is a lot easier to see where the 'line' lies.
The legal process for resolving such cases at the Land Registry does exist - it is called adjudicition. It is not to be recommended.