He whistles, surprised and impressed that the TARDIS is that far along. He thought it might be just a little room, maybe a basic console of some sort going, but he can tell Rose and the Doctor have grown her quite a lot already (not that he has anything to compare it to except the sort of 'parent' of this TARDIS and his own piece of Rift-delivered coral, and getting sidetracked). "Very nice. Right, screwdriver!"
He holds it out to the Doctor, though he looks curiously to the device while he's doing that.