The game is not only about big fights, so I don't see being "the thaumaturgy guy" as that big a problem. There is no limit on what you can do with thaumaturgy, even on short notice, evocation can't even begin to compare to that. Plus, as Harry always points out, it is about preparation. You can take some enchanted item slots and create things like Harry's kinetic rings to give you some extra power in a fight, and you will still have a lot more left in you once the other evocator has slung his last spell.
On the other hand, it is up to him how he sets the focus on his wizard. For evocation and thaumaturgy combined he got 4 focus item slots, put them all in evocation (might not be possible, but it's still 2 slots then), get more refinement on evocation and you can get a whole lot done. Of course then you are more of a combat wizard, but that seems to be the way your player wants to go.
Maybe in the first adventure, try to find some ways to give thaumaturgy a greater value, so the "real" wizard can shine without resorting to evocation only. Because thaumaturgy is what makes a real wizard, in my opinion. It does not only have to be search spells or things like that, there are a lot of ways to get creative. And of course as mentioned above: preparation with thaumaturgy will trump evocation every time.