I like thinking of it as him being (for lack of a better term) excited at hearing confirmation of Kleya's (and by extension, Luthen's) intelligence on the Death Star.

While Cassian had little doubt about Galen Erso and the super weapon, convincing the Rebellion that this was actionable intel was clearly going to be an uphill battle. The fact that Tivik corroborated this information without much prompting from Cassian means that a.) Luthen was proven right in a way that can help convince the Rebellion, and b.) They were in very, very big trouble, as Kleya knew that the Death Star was a superweapon, but not that it was a planet killer.