I'm so sorry for your loss, and I can feel both you and your furbaby's pain.
How long are you away from him everyday?
Whenever I leave the house, I turn radio on which helps.
You can also try to call your house phone and leave a voice mail(to your furbaby) every couple hrs so.... which my friend does, and it works.
You can also try window decorative film(home depot) which will prevent him from seeing too much?
PS: I think your neighbor should know that they are provoking him by staying in their front yard all day.... maybe, you can ask them if they can cut back a little?
