Some thoughts...
Is any part of his land crossed by a public footpath? If so, his 'skittish' horse might be a danger to the public and of interest to the highways dept. Does he have 3rd party liability insurance covering any footpaths?
Is he operating an equine business, eg stables, livery, etc. ? These 'skittish' horses might constitute a danger to customers and be of interest to his insurers.
Does he ride on public highways? If so, the 'skittish' horses might be a danger to the public. If they shy at aeroplanes, what are they like with traffic?
In my experience, horses are designed to run away from sabre-toothed tigers and take very little notice of aeroplanes, even low fast jets. Show them a hedge with a paper bag in it and it's another matter! Perhaps the horses in question are basket cases, so perhaps you could note if there really is a correalation between aeroplanes and 'skittering'.
Consider keeping a diary for a day or two using volunteer observers - that's what the council will ask him to do.
Of course, they may just be fit bored horses with no excercise or training to occupy them. Even then, a couple of weeks should see them adapted to the environment, unless something else is wrong..