"I will endeavour only to touch your suit when I know that world isn't at stake." It might be good for Tony's ego if he did shut it down one of the times he was trying to show off. At any rate it certainly wouldn't hurt anyone if he did. The Iron Man suit was a dangerous toy, and Ianto wasn't certain how anyone had come up with it in 1964.
Then again Tony Stark was a genius, despite his often juvenile behaviour.
Ianto had noticed his drinking a few times, but he pushed it off as just part of the era. People drank in the office, and unless it began impairing someone's judgement, he wasn't going to worry. It wasn't his job to be a nursemaid any more.