explain very politely to him about the privacy thing, ask him if he has an alternative solution for your problem and see what he comes up with. i don't think you're being unreasonable wanting your privacy, and if he's at all reasonable he'll come to a compromise with you. always try and keep it sweet with your neighbours, nothing worse than living next door to someone you've fallen out with.
but... if he won't compromise i don't think there's much he can do if you erect something, in your own garden, to stop the view from his garage window.