It was a good thing Steve was so close because there were many times Tony needed to be grounded but very few times that he knew it and it was reassuring to have someone there for the rare occurrence. Without relieving Cassie of his unblinking stare, he reached out and took Steve's hand, squeezing his fingers until he was reasonably sure he wouldn't have some kind of episode. It broke down like this: hang out in Central Park (without you, in fact admitting a notable preference to being alone than with you) until Billy (who didn't specifically clear his absolutely unremarkable schedule for this special occasion) gets off work (which Tony was fairly certain he was fired from anyway), then call up a friend (unnamed, possibly out of shame, likely male, unemployed, and socially damaging) and see if they wanna (not prearranged, like Billy, out of respect for her birthday, but just on the off chance that they might appreciate her company which they surely did not deserve or understand the worth of) go do something (also unnamed, see notes).
"We're not busy," Tony pointed out, flicking a finger between himself and Steve in case she didn't quite catch what he meant by 'we' and she needed extra help focusing on the company. That was when it dawned on him that he was holding Steve's hand and quickly let go.