The enthusiasm (or what looked on the surface like enthusiasm but was rather more likely anxiety burning off with jittery purpose) made Jarvis snort softly and hide a smile. It wasn't as if he had to hide it. No one was around to see the fondness, which was just as well. They were neither very good with emotion, and things were already fraught enough simply by the introduction of his new voice. Jarvis hummed and flicked the screen back and forth, from calendar to keyboard simply to relish the trick of it. Having seen some of the impossibly complicated things Tony could produce, he appreciated this simplicity. Easier to use it when he didn't have to fuss too much.
If there was a niggling worry about Tony near the bar sans supervision- and there was, even though the other man knew far more about drinks, specifically their taste and production- Jarvis set it aside. He'd never worried much about the consumption of alcohol. Food and drink mostly came down to texture for him, with taste a dim awareness owed more or less entirely to his sense of smell, and that meant Tony couldn't go too wrong.
He was surprised, and almost gratified, when Tony returned with beer. It was a restrained choice, earning a smile as he picked up the glass and held it aloft with one hand while the other carefully manipulated a return, "And to you, sir." For many reasons. For thinking to make the gesture at all, never mind spending the time and effort when he undoubtedly had far more pressing projects and demands to meet.
Drinking was an exercise in caution. Jarvis had learned not to make a mess, but that lack of tongue could lead to some unpleasant moments in consumption, and that was nothing Tony needed to see.