This is pure speculation based on THAT scene from the trailer where Tony looks at the photo.
Title from Crossfire by Stephen.
Tony stands outside May's apartment in Queens, his hand poised to knock. For a brief, desperate moment, he wishes Pepper were with him instead of back at base, but he knows that this is something he has to do alone. She had offered to come, and he had almost said yes, unwilling to leave her now that he'd finally got her back, but he owes it to May - to Peter - to do it by himself.
He takes a deep breath and knocks before he can change his mind. He barely has time to compose himself before the door in swinging open, May's lips forming Peter's name before she realises who it is. Her eyes harden and she glares at him with such ferocity that Tony almost shrinks away, wanting nothing more than to leave. But he knows that's not an option, so he steels himself to face her.
"May," he greets, barely able to look her in the eye. "Can we talk?"
"Where's Peter?" she asks, folding her arms and standing her ground. Tony flinches and looks at the floor.
"Maybe we should talk inside?" he suggests, but May just sniffs and shakes her head.
"Most of my neighbours are gone," she says, and Tony doesn't need to ask if 'gone' means that they've just gone out for milk. "Now. Where. Is. Peter?"
Tony blinks, his eyes suddenly wet. His hand shakes when he lifts it to brush the tears away before they can fall and his voice trembles when he speaks. "He wasn't supposed to be there," he whispers.
"No," May agrees. "He wasn't. He was supposed to be on a school trip, but then I get a call saying that he disappeared off the bus. Next thing I know, half the city is turning to dust, and Peter doesn't come home." Her voice cracks and Tony almost reaches out to her, but catches himself before he does, knowing the gesture would be unwelcome.
"I'll tell you everything," he promises. "But maybe we'll want to sit down."
She hesitates a second then nods reluctantly, moving out of the doorway to let him in.
When they're both settled on the sofa, May clutching a mug of strong coffee in her hands, she levels that glare at him again. "Talk," she says, and Tony does.
He tells her about Peter getting caught up in the fighting and ending up on the ship with him and Strange. He's not surprised to find out she knows about Spider-Man; kid was always terrible at keeping secrets. He tells her about Peter coming up with the plan to save Strange and about the arrival of the Guardians. He tells her about Titan and how they almost beat Thanos. And he tells her about Peter falling into ashes in Tony's arms, terrified of what was happening to him.
When he's done talking, they sit in silence for a few moments before May gets up at leaves, wiping surreptitiously at her eyes. Tony stays on the sofa, nervously bouncing his leg up and down. Eventually, when the wait becomes too much, he stands and does a few laps around the room. The apartment hasn't changed much since he was here last, and he feels a familiar guilt at the memory. He should never have dragged Peter into all of this.
He's on his seventh or eighth lap when he spots a photo standing proud on one of the shelves. He looks closer and recognises it as one of him and Peter from years ago, back in happier times. They'd taken it to give to May, to keep up the pretense that the internship was real. He picks it up and studies it, his heart aching at the memory of Peter's wide-eyed, boyish grin. He'd been so happy, even though he knew it was all fake, and Tony wishes he could have saved that innocence, wishes Peter could have remained that boy in the photo, if only for a little while longer.
"He always looked up to you, you know?" May's voice behind him makes Tony jump; he hadn't heard her come back. He carefully places the photo back on the shelf and turns to face her.
"I didn't deserve it," he mumbles, the guilt returning ten-fold at her words.
May hums, looking away from Tony at the photo. "I don't know about that," she says. "But I do know he seemed more...focused, after you came. Before that, he was, I don't know, on edge. Erratic. I think you gave him a purpose."
Tony stares at her, stunned. He knows that May will never be his biggest fan, but hearing her speak warms him. He reaches out and gently places a hand on her shoulder - she stiffens at the contact, but doesn't pull away, which Tony is thankful for.
"I'll get him back, May," he promises. "Whatever it takes, I'll get him back."
She looks up at him, eyes shining and full of reluctant hope. "Don't make promises you can't keep, Tony Stark," she tells him. Tony winces; that's a lesson he should have learned a long time ago, but he refuses to retract what he said. He smiles at May and heads to the door, but she catches his arm before he can leave.
"Thank you for coming," she tells him, and he can see genuine gratitude on her face. He nods, then walks out the apartment, making sure he's well away before he lets himself cry, too.
Later, on the way back to the compound, Tony makes a silent vow that this is one promise he will do anything to keep. He will bring Peter back. Whatever it takes.