I Fucking Miss Tony Stark - Tumblr Posts
Here to remind you how Tony was unfocusing his gaze 'cause of the pain he felt while probably struggling to recognize what was happening around him and who was the person in front of him, but as soon as he heard "it's me, it's Peter" he immediately focused on him.


And while he couldn't find the force to speak, my theory is that he didn't do it also because he didn't know what to say to make the kid stop crying, to make the pain go away.
He knew what to do with Pepper and with Rhodey, their reactions were so part of their characters: Rhodey and Tony didn't need to share a word, they have been friends since ever and they understand each other without the need to talk; Pepper and Tony both smiled at each other, to reassure the other that everything and everyone will be fine.
With Peter? I think Tony not speaking was also in character. He didn't almost speak when Peter was dying, he didn't know what to do. He tried to comfort him when he said "you're alright", but as soon as things got worse he just stood there in shock, without never taking his eyes off him while Peter was crying and begging him to do something.
The same happens here:


In the photos you can't catch it very clearly, but if you watch the complete scene you can see how Tony is not taking his eyes off him and -even when Pepper is there and is trying to take him away- he's trying to calm him down with his eyes, because that's Peter, the kid he lost and that he found five years later just to lose him again. I don't know if I'm the only one, but I almost saw panic in his eyes, it's like he was scared about Peter not being okay and about him not saying a word again. That's why he whispered kid once he wasn't in his gaze anymore, he wanted to see him one last time, to make sure to reassure him like he did with Rhodey and, soon, with Pepper. To make Peter sure that he will be fine even without him.