Hi,
It sounds like the little guy is doing better. See if you can find a starting point for the ACV, like one or two drops in his water bowl and build up from there. Even a small amount of ACV will change the PH in his GI tract and help him. Some ACV is better than no ACV at all. Our birds get ACV in their water several times a week and seem to really like the taste. I will give them extra any time I see their poop is a little off. Jacob may like the applesauce warmed to 39-40 degrees Celsius. If he like the applesauce you will be able to sneak a drop or two of ACV in there a bit later. Keep him eating offering his favorite foods. I would not let him drink to much liquid at one time as you want him to keep food and water down rather than have him vomit. What are his favorite foods?
Keep up the good work, I know having a bird not feeling good can wear on you.
gary