He keeps saying il just sleep in my car(obviously I can't let him do that )
Why? Are you his mummy? His warden? His minder?
He obviously knows exactly how to manipulate you.
He's a grown man, presumably with a job. He can sort his own roof out without reference to you, or your roof.
But I know once he is in mine he won't leave.
So ... you know what he's like ... but ...
I feel totally trapped into letting him stay now
YOU ARE NOT TRAPPED.
It is really, really simple.
Not easy, as you have an odd notion of owing your useless wanna-be-cocklodger a living ... but brace yourself, & do it:
"I don't want you moving in with me. You need to take responsibility for your own life."
You KNOW how he is going to respond. He will whinge about just wanting a few nights to sort himself out etc ... he will lie, manipulate, guilt-trip & use you to get his own way.
At this point, tell him how unattractive his behaviour is, that you have no wish to house him, & in fact no wish to even be with him, as you deserve an adult who knows how to rent his own home.