This must have been coming as quite a surprise to a boy (although, actually, he was probably the same age as Luke, about) who'd been raised as a Stormtrooper to learn that he might have it within him to train in the ways of the Force. Luke had been surprised and in disbelief and he'd come from an upbringing much more humble if not boring.
"We can see how it goes," Luke agreed. A twitch of a branch from an untrained force user was actually fairly impressive. "You're alright, Finn. I know it's a lot, but with a friend like Rey, you're going to have to get used to the encouragement."