He doesn't know how to say that he's lying to Tony currently, but he also doesn't want to tell him and blow up whatever friendship he's built with the Soldier, especially if it turns out untrue for their world.

And he definitely doesn't think Tony will take all of this rationally and reasonably.

"The truth was something that hurt him. I thought maybe I could spare him." He now specifically says him to distinguish from this Tony. Maybe then he won't think this lie pertains to him.