Only you know your neighbour and what your current relation ship with them is,I would recommend a gently gently approach, ask him if he would mind letting you know next time he sprays as it effected you. He shouldn't allow drift onto your property is the bottom line. He should have proper training, do you think he has ? Look at the hse website, agriculture spraying, and read the code of practice on there.
But...try desperately to keep it friendly, falling out with neighbours can be a miserable experience. And farmers in particular can make life very unpleasant whilst staying within the law.