Tony's a little bit gobsmacked, and it shows. There's a lot of history there. More than even Steve is fully aware of. The legacy of the serum and Captain America and its affect on Tony and his father's relationship. The role the serum played in his father's death. Making peace with all of that. It's all swirling around in his mind as he accepts Steve's gift.
He's actually quiet for a moment. Well done, Steve. But then he manages to convey that he's honored by the gesture and promises to keep it safe.