You should really talk to a housing lawyer in Massachusetts. Most of the time, they'll give you a few minutes of their time for free if you call for a consultation. I did this once after I was in a car accident -- very helpful ~10 minute conversation on how to handle that, and I didn't actually need a lawyer in the end. https://www.usa.gov/legal-aid

This situation you're in is a bit of a grey area, as you've found. Ignore the politically motivated trolls whining about how tenant friendly MA is. He's not a tenant. Worst case scenario, he's a lodger. You're not a landlord, you're a homeowner with a deadbeat roommate. Best case scenario, he's merely an unwanted house guest and you can get the cops to remove him for trespassing if he doesn't leave. A lawyer would help you figure out exactly what he is and how to remove him legally. The definitions overlap somewhat.