Tony would be horrible to a pre-serum/deserumed Steve
Tony mocks people as a form of entertainment. In the Avengers, Steve is two weeks out of the ice and Tony is making jokes about Steve's trauma, rolling his eyes at Steve's attempts to fit in, and outright verbally attacking Steve. (And that was all when scepter wasn't even in the room.)
Tony is also dismissive of people who want him to like them or even help him, such as Harley Keener and Ho Yinsen (in the flashback) in Iron Man 3. So why would anyone think Tony would do anything except mock Steve in this scenario?
***
"We'll figure out how to fix this, Steve," Bruce said as he studied the readings on a nearby screen. He frowned at the list of ailments being detected and continued, "In the meantime, maybe you'd like to rest?"
"Better listen to him, Chibi-Cap," Tony said as he strolled in. He continued, "We don't even know where to start putting the super and soldier back into the...you."
Steve shook his head and said, "I'll be fine. We need to adjust our plans for protecting the summit. Whoever did this wanted me out of action, so we'd better make sure we fill any gaps in the security."
Tony smirked and said, "Well, we could hide you in the podium, or in the keynote speaker's pocket. Not sure what you'd be able to do if they were attacked though. Maybe bite the bad guy's ankles?"
Bruce considered pointing out that Steve was not THAT much shorter than Tony, but kept quiet instead and hoped Tony's "humor" would soon run out.
Tony was circling Steve, who was focused on the blueprints of the conference hall. Tony said, "I've seen the photos, courtesy of dear old dad, but I never really got the whole picture until now. How the hell did anyone let you into the Army? Your backpack would've been taller than you. Were you some kind of mascot?"
Steve's jaw tightened, but he simply moved on to the list of patrols and procedures for the summit, making changes.
Tony continued, "I could see that. Little squirt trying to keep up with the big boys. You were probably too adorable to resist--that's why my dad and that crazy doc made you into the perfect angel poster boy of World War II."
Steve stilled, then walked over to Tony. Steve said quietly, "Doctor Erskine chose me because he thought I was the right person for the project. And because he knew I didn't like bullies, no matter where they were from. I still don't like bullies, no matter who they are. So maybe find something better to do with your time."
Steve looked over to Bruce and said, "I'm going to coordinate with Natasha. She'll take the lead on security if I'm not...myself by morning. Please keep me informed of your progress."
When Steve left, Tony turned to look at Bruce and opened his mouth. Before he could say anything, Bruce said, "Just shut up, Tony. If it's not about helping Steve, I don't want to hear it."
Bruce straightened up and realized that it wasn't Captain America who inspired people to stand up and speak out. It had always been Steve Rogers.