Hi J.,
I'm 54 now, but none of mine could tolerate cows' milk when they were little, and the middle one coldn't deal with soy either. I was working with the first two, and wasn't very successful at nursing them, so it was a real problem. Ultimately, I went to my mother-in-law (My mom was gone.)and asked how to make formula from scratch. I used goats' milk and it was very successful. Because I have allergies and they were plainly sensitive, I didn't try weaning them until they were almost a year old. Of course, they got all the recommended vitamin supplements as well. Then I started very slowly. I found my allergies with an elimination diet, which means you stick with foods that you aren't allergic to until your system is cleaned out, then slowly add back other things, while watching for a reaction. Essentially this is the same as starting a baby on his new foods. The least allergenic things are supposed to be rice, lamb, and apples. The rice cereal is a surprise, and makes me wonder if there is some common preservative or additive in them as they are fairly processed foods. You could try making the foods from scratch, so you know there are no additives, etc. And I wouldn't worry if he needs to nurse for a month or two longer. Mine eventually outgrew it, and they eat a normal diet, including the standard junk that I wish they'ed leave alone.
There could be an issue with the celiac disease as well, since blood in the stools is a classic symptom, I'd particularly look for a gluten additive to the foods, perhaps to stabilize and/or thicken.
I think you're smart to be checking with all the sources you can, you're doing a great job.
L.