"Oh, shoot," Ted comments (loudly enough to announce their presence) as he enters the living room, immediately switching into cheerful dad mode. "Beard and I had a bet going about which one of you was gonna mention Hawkeye first, and I missed it. Which one of you was it?"

One thing that makes it a lot easier to slip into that cheerful mode is his immense relief at seeing Kate and Henry get along. Especially after the conversation he just had, he's glad to see he's got another reason to have faith in Kate and this relationship. He had plenty already, but now Michelle gets to see it too. (Not to mention hearing Kate speak kindly of him like that.)