John..perhaps this may help you:
http://www.french-property.com/guides/france/building/planning/opposition/
Hello John.
How many times have I heard this type of story over the years?
The only thing I would like to mention is that you immediately check your house insurance policy to ensure that you have Protection Juridique cover. This will enable you to obtain free legal advice (up to a point) should the problem actually get that far.
I would also advise that you do not put any faith in the Mayor or the planning department until, that is, you are sure they have acted completely independently from neighbours son (e.g. they are not in the same rugby team, or cousins..).
Then, assuming you have sufficient French language skills, go see the neighbours son... maybe it won't be as bad as you think.