I'd have been so shocked I think I'd have said what the fuck do you think you are doing.
Something has happened at his parents. NO ONE thinks you can just move yourself into someone else's house without discussion.
Why on earth are you worried that asking him to move out will end the relationship? If you were able to recognise his behaviour for what it is you'd pack his bags, leave them on the doorstep and tell him to fuck off and never come back. He's a piss taking, cheeky fucker, he needs to be removed from your life.
Don't waste time thinking about how to put it nicely or try to explain yourself, he'll only see that as an opening to a negotiation. "I don't want to live with you, pack your things and leave", is all you need to say to make your position crystal clear. Where he goes is not your concern, his housing is not your responsibility, don't engage with any discussion about this.
The fact you are asking for advice and are worried that not allowing him to move in will end the relationship says you have poor self esteem and no boundaries. You missed your opportunity last night to show him you aren't a push over but all is not lost, you can still do it today. It is your house, only a real cheeky fucker would think they could move in without discussion.
If you don't kick him straight back out I predict you'll be back here in a few months complaining that he does fuck all around the house, constantly criticises you and pays nothing.
Get rid of this would be cock lodger today, the end of this relationship is no loss. Your feelings and needs are not even on his radar why would you put up with that.