So, from her point of view, she may already be toxic and awful. But she is also a mother who believes her son when he says you are abusing him. While I wouldn’t recommend a parent try this tactic, many do. As far as she is concerned her son in unhappy in his relationship and abused.
You already know why he didn’t stand up for you. Because she would have come out with all the things he has said about you abusing him. And maybe some worse things. You know this. You know, your relationship is awful because he tells people you are abusing him. Your relationship is a non starter. With or without this incident.
And yes, he likely moved in with you because it was easy. You say he didn’t pay anything, or paid very minimal amounts. It couldn’t be any clearer that he is in a relationship with you because of those benefits to him. Not because he wants to be with you.
You don’t need him to tell you he doesn’t want to be with you, for you to end it and stop him taking the piss out of you and using you.
His mother has done you a favour. I know you told him to collect his stuff, but don’t let him talk you round. Why he is the way he is, isn’t the problem. The fact that he is using you, is the problem.