If someone really truly loves you there will be no one else in the picture at all. You should be able to tell if someone truly loves you by the way they look at you, their actions, they are there for you no matter what happens, and you should be able to feel their love for you.
Well hun, if he loves you, then why would he love someone else? Or why would he love you if he loves someone else? Just think, if he truly loves someone, he wouldn't love another person even if he shows it or not. So, he might be lying, sorry to say.
Is it really known that he in fact loves someone else... Most times if someone is in a relationship its because they want to be and despite if there was a possibility of someone else they made their choice by staying where they are.