、。
You've really started throwing some good pitches, Keiichi.
Literal
Good ball [object-を] come-to-throw [reflective-な], Keiichi-kun.
~ようになる marks a gradual change of state or newly acquired ability — 'come to do X,' 'start doing X,' 'become able to X.' Paired with 放る, a casual/colloquial verb for 'throw' that is especially idiomatic in baseball for pitching, this sounds like a coach or senior praising a junior's visible improvement. The sentence-final な here is reflective or self-confirming — almost talking to oneself — distinct from the prohibitive な that attaches directly to a dictionary-form verb to mean 'don't do X.' The vocative 恵一君 at the end identifies the addressee; the grammatical subject of 放る is elided, as is normal in Japanese.