Is my friend being abused?

Hey. So recently my friend and I (we're both girls) have noticed that our other friend Calvin's been acting strangely. We've come to think that maybe he's being abused or neglected at home. I'd like to list a few of the strange issues or events we've seen recently, as well as a few general facts:

Calvin is twenty years old. I realize that at this point he's legally an adult, but that doesn't mean he can't still be abused. Anyway... He's very kind and intelligent but he does poorly, has a tendency to slack off, and is generally considered to be unintelligent by most of our peers.

He's of about average height but he's very, very thin. To the extent that if you hug him you can feel the bones up against the skin of his arms and feel the notches of his spine against his back. He is not an athlete, nor does he do any particularly strenuous work.

From what we've seen his parents hardly feed him. His meals are very small, and they make him pay for his own food. If he can't pay, he doesn't eat.

He has severe dental issues but his parents won't pay for him to see a dentist, despite being middle-class and working and being capable of affording the cost.

He was adopted from Southeastern Europe at about the age of three, and it was recommended that he speak to a therapist, simply because he came from a bad place. They don't bring him to see anyone.

He hasn't seen a doctor or a dentist in years.

They don't give him any money for college and he is going solely on loans. He spent the entirety of his summer working two jobs to pay for his food and his textbooks, and rarely took a day off at all.

He sometimes says that no one cares for him or loves him, and will often call himself stupid and unintelligent and a disappointment, even though none of these are true.

He's posted videos of family fights on his tumblr account. Not to say that it's odd for families to fight but...it's odd to post fights on tumblr.

He's bizarrely devoted to his father and to his two sisters but his relationship to his mother seems strained at best.

Recently we had him over and he couldn't sit down, though he wouldn't SAY that he couldn't sit down. He couldn't sit, had trouble climbing onto one of the beds to watch television or use his laptop, hid in the closet when someone knocked on the front door, acted nervously, wouldn't stop looking at his phone, insisted that we stay with him, snored very loudly when he's usually relatively quiet, and was acting nervous and overly friendly for the duration of his stay. He wouldn't talk about what was bothering him, but we know he hasn't been going home these last few nights.

If our suspicions are right, it's neglect at best. But it COULD be psychological, physical or sexual abuse. Or all three.

How should we deal with this? We know that he's legally an adult but we can't just ignore the situation. If it's nothing, it's nothing, but if it's something it can't just be ignored. We've not told anyone else apart from our other roommates--in case he needs somewhere to go.

We're going to continue looking into this, but I wanted to know your thoughts. Real issue, or are we imagining things?