My New Hampshire had sour crop before and miconozole worked wonders on her, so that's all I can suggest.
I'm sorry I don't have further advise for you, but these people have helped me greatly in the past...
@azygous
@Eggcessive
@speckledhen
Does his crop feel sort of like a water balloon? How does his breath smell? If it smells slightly or very rotten, give him miconazole twice a day for seven full days.
Here's a link on crop problems, and the best of luck with your rooster...