Who's paying for the Internet? If it's him, you're better off paying for separate service. If it's you, then you need to put your foot down or lock him out of it, for the sake of your own computer and data.
The best bet is to make sure everything is up-to-date on your computer and block his computer from accessing yours. Back in the day, cable routers were notorious for security risks, and I don't know if that has changed so you should be very careful.
Best practice to avoid security risks: turn off your computer, or at least never use the Internet. If I'm right, that idea doesn't appeal to you, so here's a common trick used in business. Have two computers - one for Internet and one for actually doing everything. You'll need a secure way to pass needed info back and forth, of course, but if you do it right you'll never have to worry about sensitive data being stolen (unless you leave it on the Internet computer or transmit it over an insecure connection - which is what the Internet is full of).
Namaste, Peace & Love,
If I have frustrated you, then I must be a student. If I've imparted information or a skill to you, then I must be a teacher. If I've helped you, then I must be a volunteer. If I've touched your life, then I must be happy!
If you had to choose between saving just your family, or saving 10,000 GOOD people (but not your family), what would you choose?