I think I'm falling in love with a guy in a casual relationship
What happened that made you fall in love with someone who is in a casual relationship?
Recently we spent a lot of time together and I was impressed by him
As long as the guy your in a casual relationship is single, it's totally fine to fall in love with someone.  If you two are meant to be together, you will be together so don't rush it and see where it goes.
Yes we were supposed to be together...He's my rebound XD
I am glad you have found someone you have love for. I hope you can make this relationship exclusive so you have the best chance at success.