What indirect characterization do we infer about him? What do we later infer about Bobby Martin’s action of collecting “the smoothest and roundest stones”?

From the text, we can infer that Bobby is a leader amongst the school-aged boys, as they follow his example. We can also infer that he has a plan for how the stones will be used. We can later infer that by using the smoothest and roundest stones, they will do the job without producing a lot of blood (jagged stones would cut the skin).