My question involves a roommate in the State of: Connecticut

So My fiance and I signed a one year lease with our roommate. Things have been okay until this past Sunday. He tells us he's thinking about moving out and we'd all sit down and talk about it the next day. Before we even sit down to talk he tells us he'll be out by the end of the week and later that day by specifically Wednesday. I have no idea what he plans to do about his name being on the lease.

I know we are responsible for paying rent on time but my question is if he doesn't take himself off of the lease, can be held responsible for half of rent(what we agreed upon to begin with)? Also, is it legal for him to give 3 days notice before leaving?