What do you do when someone says "I love you" but you can't say it back?

I have a boyfriend and I just met this guy and we became friends. He knows I have a boyfriend but he still doesn't care and he'll say it during conversations and I don't know what to say back so there's an awkward silence. What can I say to not hurt him? (I love my boyfriend and I would never wanna romantically say I love you to someone else except him)