A HUGE question - Is he an introvert? If yes, then what he is saying makes sense. He doesn't have a place to recharge his energy. Extroverts get their energy from others around them. Introverts get their energy from themselves.

What he is craving is alone time each day. The current living arrangement doesn't give him that.

Something for you to think about - does he at public gatherings disappear every so often? If yes, this is a coping mechanism so he can recharge. I will bet he hides in the bathroom for a few minutes. It is a common tactic for introverts to use in situations like this.

This webpage explains it simply and perfectly. https://danineteen.wordpress.com/2013/05/08/dr-carmellas-guide-to-understanding-the-introverted/

I have a suggestion - how about finding him a block of time a few times a week where you leave the house for a couple of hours so he can have his alone time. You could go to the library, the park to read a book, something. The important thing is you are giving him alone time blocks that it sounds to me he desperately needs.

Edit: Do NOT think you being with him doesn't take energy. IT DOES. Do NOT make this about you whatever you do. If you go that route, it WILL backfire on you and make things worse.