Turn off Universal Plug and Play (UNP), WPS and Remote Access.
Turn off Port Forwarding.
Turn off respond to Pings (or turn on ignore ping whichever is the setting).
Make sure both Windows Firewall on your devices and the Firewall on your router are switched on.
Set a difficult Username and Password (you might need to keep a text file of these in notepad and cut and paste them when you need to log in to your router (also keep a print out just in case you have issues with your pc and lose the text file).
I suggest a username that isn't guessable ie not relatives names, favourite places, birthdays, cars, "let me in" or similar but a name that's maybe 5 or 6 characters and a mix of Upper and lower case and numbers.
For your password, I suggest 12 to 18 characters long of random letters, capitals / lower case, numbers and symbols.
Repeat the above for your wifi password as wifi is another easy way in. Don't ever let him connect via your wifi other than a temporary guest network and restrict access to guest accounts. For wifif your own devices will remember the password and agin you can have it written down somewhere convenient he cannot find it for if you ever need to reset a device or reconnect it.
Also, don't give him access to your devices ie let him borrow your laptop or phone etc as he could discover the passwords or install logging software - good point here, might be worth virus scanning or scanning with malwarebytes to check for keyloggers etc on your devices. Not sure how literall to take the hacking bit but if he's an amateur hacker, he may have installed ways of capturing passwords on your devices.
My guess is that if he's external to your property, he's probably gaining access using Port Forwarded addresses / Remote Access and / or UNP to install his device as a permitted device.
This pretty much says what I'm saying here, but might have some more additional tips:
Enhance your home network security with our expert guide on Wi-Fi router protection. Learn essential settings and tips to safeguard your data, whether using a provider’s router or your own. Keep hackers out and your information safe.
www.cable.co.uk
Most home Internet routers have serious security flaws, with some so vulnerable to attack they should be thrown out, an expert warns.
www.tomsguide.com