We would not call him at all because it would be one of the many millions that went through projects like this for their thesis as physics or math graduates.
One of my best friends in his bachelor thesis had solved a difficult mathematical problem in planet orbits or something, and it was just yet another random day in academia.
And she didn't solve it because she was a genius but because there's a bazillions such problems out there and little time to look at them and focus. Science is huge.
True. If you stay in your domain for a very long time the people with you in that niche space are less and less and when you solve something that wasn't done before it's not necessarily a hard problem.
Yet, if some student or child achieved the same – under equal supervision – we would call him the next Einstein.