he's a user, you're well shot of him. He was keen as mustard or dates one and two, because he hadn't yet got he wanted. As soon as he had, he discarded you like a used Kleenex. He is not someone to have in your life, he has no respect for you.
don't bother explaining to him why you never want to see him again, just block and move on, he doesn't deserve an explanation.
he wasn't invested in you and didn't care about how you felt, not after 3 weeks.