I don't know how to deal with sour crop in an adult, so it would be best to see your vet.
Not sure if you know who Susanne is, but she used to have some excellent articles on dealing with sour crop in babies. Appparently she is now only making them available to people who pay for the privilege, and I don't know how one even makes arrangements to fork over some cash. But it's possible to view the old version of her website on archive.org. Here's a link to one of her articles on crop problems in babies:
https://web.archive.org/web/20160414035 ... edies.htmlThere are more if you click "Articles From the Nestbox" at the top of the page.
Good luck, I hope he feels better soon!