I am NOT going to give any advice on introducing them, since Kylie is my first cat, I have no experience with that.
But I WILL give support.

Kelly, you are trying and that is more than SOME other people would have tried. If you DO have to take him back, please know that you tried and he probably needs to be a single cat - cat.
Maybe the shelter didn't lie, maybe the people did that took him to the shelter?