I do think, especially in Henry's case, that all of this is linked together. IMO, understanding and fixing the nutritional & gut issues is critical. Why won't the hematologist respond to your B12 question? I think it could be very beneficial. Your circumstances are unique in that I can't recall any child with AA and autism in the past six years that we've been dealing with this disease. And to find a hematologist who understands both will be a challenge. But you want someone who's treated AA before or one who is willing to work closely with an expert.
Regarding benzene.....it's so hard to pin point a cause unless it happens within 3 months of the exposure. Benzene doesn't stay in the blood very long and it's usually a bigger exposure to trigger it. So small exposures over time are hard to prove. But again, in autism, there's a school of thought that some children do not elimate toxins and heavy metals effectively from their bodies.
There are some post on another forum that talks about the efficacy of treatment depending on cause but I can't recall any of the details. I think if it's a clear autoimmune trigger (like a vaccine), then they think immunosuppressant therapy is pretty effective. You can search the posts on the forum at www.aplasticcentral.com
for more info on this. But most never know how they ended up with AA. They do need to rule out genetic anemias (Fanconia, Black Diamond) as well as PNH. So make sure they checked him for them.
There aren't many approaches to SAA....it's either a BMT or IST like ATG with Cyclosporine or High Dose Cytoxan. They usually do not treat moderate AA. So if Henry is in the moderate stage, then hopefully, he'll stay there and you'll have a chance to resolve the nutritional and gut issues.