How much blood...and what color? (bright, or old and clotting? There is a difference.)

Old blood needs to be seen ASAP. Bright could be from irritation from the vomiting (IE, a scraped up esophagus) Does he have any sinus drainage issues? That caould cause nausea, vomiting *and* the blood (and be nice and easy to fix, if this happened same time last year it could even be an environmental allergy exposed to his gut through postnasal drip)

What kind of blood work has been run? Pancreatitis can cause abdominal pain and vomiting, but wouldn't show up in the gall bladder.

I think I'd go on a pure liquid diet, broth and juice and maybe gelatin (not red) for a few days. Then slowly add other stuff back in. Have you considered the SCD? A lot of people on the list talk about passing blood, and vomiting blood, and using the amount and associated comfort as guidance as to whether or not the diet works. Obviously...you'd put him on the *starter* diet. No nuts/baked goods.