It sounds like your little guy might have sour crop. It's a fungal infection of the crop that makes it bloated and often smells terrible. It's also possible that he has an impacted crop, blocking the food from exiting and making it ferment.
Apple Cider Vinegar added to the water is supposed to work very well for sour crop. Just put 2-3 tablespoons into a gallon of water and make sure he drinks some. Dribble it down the side of his beak, if he isn't taking it willingly. In severe cases antifungals from the vet may be needed.
Another thing you can do is gently massage the crop in an upward motion. This will help the crop move whatever is in it out of the crop. If you can, try and notice if he is still having crop contractions. You'll feel the crop grow very firm and contract. If he's having them, then that's great. If he isn't, you may have to do the message often.
I hope your little guy gets better soon, keep us posted!