[Tibo inc]

Do you mean you can access his network but not login to the router? Either way if you want to stop him connecting to the internet download Kali Linux on either a USB or Virtual Machine and run Fluxion. It's designed to DOS a users network until they sign into a phishing page where you gain credentials and their service will resume. Check Github or Google and you can just DOS indefinitely

I was about to post this exact thing, if you are connected to his WiFi you can phish him, but this will only give you his WiFi password, which you seem to already have

Update: Just realised I'm horribly off the mark here and yes this should give you his router credentials as fluxion makes it seem like your router needs updating.