Most Helpful Girl
No, it don't think he regrets it. Remember that when you guys had sex you agreed that it was just a casual thing. As for him, he had no right to be mad at you for seeing another guy. He was the one who wanted to keep things in a casual way, not you.
It seems like he wanted you to think about him in an exclusive way, while he was not willing to do the same for you.
Anyway, I don't think he is serious about you.
You should find someone else who appreciates you more.