Is this sentence correct grammatically?

Yes, but it is informal. Although the preposition before him is often dropped in speech -- and in informal writing -- more formal writing would conclude the sentence with either "for him" or "to him" -- depending upon the slant you wish to give the sentence.

You might also say "What good resulted?" or if you wish to be really formal "How did this benefit him?"

These are all choices of style. Who is your audience? What kind of writing is this? You want to choose a writing style that fits your purpose and audience -- an academic term paper, thesis, or dissertation needs to be more formal than a personal essay. Since I don't know the purpose of your writing, I can't advise as to whether it is too informal or not -- it may be just right.