I tend to not deviate too much from canon, if at all really. Theo being a relatively silent sort wouldn't cause something like that.

Poor Alphinaud indeed. Theo would have carried on more casual conversation with him. Correcting details of stories. Likely talking about the differences between his sister and himself. Because he likes kids and well. He's kind of attached to at least one person at that point.

When asked questions, he'll answer at least. Maybe not too much about his past ( but he would talk about some of the places he'd been before Eorzea ) where his family and home village are concerned. Definitely nothing about Viera and how they are.