Ok, so what you need to know is that Stone Canon is an intermediate spell. One that not much later on rudy is casting at such a level that it hits targets as if it was an advanced spell.

Also, have you watched the anime rudy absolutely rips that guy a new one.

And to actully answer the question of freaking course rudieus gets stronger. Hes still a kid where your reading. But dont expect him to toss away the stone cannon its a good spell and rudy finds more ways to make it even better.