The exact details of your respective properties may matter. As a general rule, though, it is not illegal to photograph anything that you can see from a place you have the right to be, other than photos of things for which privacy is generally expected (e.g. taking a photo of the neighbor getting out the shower with a telephoto lens through his/her window). If the photo might subject the person being photographed to embarrassment or humiliation you'd do well to carefully consider whether that may end up causing you to be sued, perhaps successfully, for violating one of your state's privacy laws. Before you escalate this war with your neighbor, it'd be a really good idea to consult a MA attorney regarding whether there is any legal action you may take about his camera and what risks you run doing the same thing.

Note that if the camera also captures audio federal and state wiretap laws will apply and, again dependent on the facts, might make one or both of you in violation of the law for audio recordings you capture with the camera.