Let him put up the barb wire fence.
Don't panic already. Go to your county eng. and zoning and complain that this barb wire fence IS dangerous (and it is) for children and animals. It does not matter that you may not have any children and or animals...but someone may come visit that has them, RIGHT??? He HAS to have a permit for this. If warm outside sit outside in your rocking chair and enjoy watching him put it up!!! Get a dated digital photo of when he is putting this up. To prove it hasn't been there in your counties "grand fathered" limits if any.
Then...when the zoning guy comes out to make him immediately take it down...sit still and watch him do that too.
Heck...I had my grand dad with old farm land in N.M. his residence as well as across the street from his res. where there was not a sole around. This he loved. The lot next door and across this country road had sold and some city folks were building there to move to the country. He watched the builders from his porch while sitting in his rocking chair. Week after week watching this house go up right? Water lines, electric and everything. He waited patiently!!!
Only...it was on the wrong lot, grand dad owned the new house, free and clear!!! PATIENCE IS A VIRTUE. In the a.m. go to your county's zoning or where ever that your county "building permits". Do not call them but go down there in person and make a formal complaint.
Barb wire IS dangerous! They even have height requirements and everything on actual real fences too... You cannot just "put up fences" let alone...barb wire.