First of all talk to your boss so they know what is going on and can support you.
You are not breaking any laws by parking outside his house. He is being an ass.
In the mean time, start a diary - as tedious as it is. Note the dates you are not parked outside your house, and record where you are parked.
Keep a record of any phone calls, letters, comments from this man.
It doesn't need to be a novel - just the bare facts and dates.
If you can see from your written record that he is doing enough for you to consider it harassment, take it to your employer.
If your employer doesn't support you as he should, think about seeing a solicitor to threaten an injunction against harassment,
Be warned this is a very last resort - such action will irretrievably end any possibility of a good relationship with your neighbour, although it should stop the harassment.
In a nutshell, don't let him get to you. Keep a record. Talk to your boss.