Also too you want to make certain you have tightened up your network security. You need to know how to access your routers control panel. For example if you have a linksys router it would likely be http://192.168.1.1 but you can also use the ipconfig /all command from the command line from your computer to see what your default gateway ip address is. Its been a good while since I did this kind of thing but I think these are the steps necessary. If I leave anything out or have anything wrong surely someone will let me know.
Once in the control panel their are some things you can do to tighten security. The first thing I would do is change my routers password to insure I am not using the default password. If it is still set as the default likely it is admin for the user and the password is blank. I believe this works for both linksys and dlink routers but could stand corrected there. If you can't get in you might have to push the little reset button on your router and reset it to factory defaults and start fresh.
Also too, you can actually make your router potentially invisible to him. Of course if he has a snooper it won't help you but if not he won't see your router. To do this you would deactivate SSID broadcast. Also too the SSID broadcast is set on a default, For example on linksys it says "linksys" I think along with the model number but maybe just linksys can't remember. I would change the SSID name to something else but not something that tells anything personally about me. Maybe "bigbrotheriswatching" or something lol. Then if he says something maybe tell him (or even drop casually) I am tired of the internet so I let my connection go") and perhaps try some misdirection to get him off your tail a little bit, so to speak and just maybe the drop of your unique identifier in the SSID might misdirect him into thinking you don't have internet anymore.
Another thing I might do is to turn off UPnP you likely do not need it and it is a potential security issue.
Of course I would use WPA over WEP as WPA is supposed to be more secure.
I would use MAC address filtering to mitigate anyone else surfing on my network wirelessely. To turn on MAC address filtering you need to know the MAC address for each wireless device on your network. To determine your address at the command prompt type ipconfig /all and look for the physical address. It is six hexadecimal numbers like 00-24-92-CE-5F-49-9E. Once I have this informaiton then from my router control panel I go to the filters section and apply MAC filters, entering the name and MAC address for the computer and each box in my house that connects wirelessly. If you are connected to your router though with a network cable MAC address filtering won't apply. But at least with MAC address filtering it would potentially stop anyone from wirelesseley using your internet connection unless thay have been given authorization via the filtering.
And lastly, I would update my firmware.
Of course, this isn't my area of expertise but I think these items will help you.
But too what concerns me is beyond him having access to your internet. History shows us clearly that people with obsessions like he is seemingly showing should be seen as a warning of potential escalated issues, to put it nicely. At the very least, you need to go to the Police and at LEAST get it into the public record by filing a report. Agreed, there is probably nothing can be done at this point but at least it will document it properly and in the event you turn up I'm sorry to say, disappeared we'll all have a trail to follow.
It wouldn't hurt too to take a self defense course, pourchase some really good mace or stun gun that kind of thing. Look- the important thing is you must take this very seriously and hope for the best but expect the worse. Don't be naive. I am not a psychologist or other type of expert on this matter by any means but I do watch CNN. He has shown what appears to me as the classic signs of an obsessive personality according to how you have explained this. Clearly if we watch the news we see examples unfortunately all the time of situations like this where people saw the warning signs, didn't know better or didn't do anything about it and ended up the worse for it later. So at the least seek professional assistance that would be the best advice anyone could give and something you should listen to and not tomorrow, today.
(But also too, all that said, what Deepsand says too we should take note of. Perhaps he is just simply obsessively curious or just thought he had an edge on getting inside your pants, um head, and isn't really a bad or evil person. But all the same, like I say, hope for the best, expect the worse and prepare accordingly and at least you are covered to the greatest possible extent.)