Hi Ssis ........... I guess this is a drain used by other properties and not just his?
Even if it is, it may not have been adopted by the local Water Authority. If you rang them and asked for the Mapping Dept. (might be different name where you live), they will have a drain map. They can tell you if your drain is on the map. If it is, your neighbour is being a prat. As said here, they would just dig it up if needed.
If not adopted, then you might have to uncover it if you need rodding access or whatever.
Either way .............. Prat.