I mentioned lead poisoning because I don't know anything about your life, or his "eating" habits. For all I know you live in a house with flaking 100 year old paint peeling off the walls, and Oz is a sneak who eats anything that hits the floor. It may not apply to your situation, but I have no way of knowing that.
Are there grains in his diet? Given that it is a specialized diet it may not. Wheat is of course the one most people blame, oats can also be problematic, rice should be fine. Anecdotally, the pathologist where I got my PhD said ~80% of the animals at our facility showed signs of celiac's, this represents a sample size of several thousand animals (over his decades long tenure at the facility), our animals were on a diet that contained wheat, and was provided by Purina (they make most lab animals' food). These weren't dogs, but 80% is a huge number.
http://marycoordt.com/wp-content/uploads/2012/06/stages-of-villi-800px-Coeliac_Disease-800x311.jpg
This isn't about celiac's per se, but the illustration shows what the progression from a healthy to a damaged intestine looks like. The difference in nutrient absorption can be huge.
I am by no means an expert on pro-biotics, but if possible you might want to use a rotation through a few different bacteria options, staying with the same choice could hypothetically lead to a gut that doesn't have a good balance of bacteria. Maybe switch a few times a year, if variety exists in the market.
You may also want to consider adding supplements to his diet. If this is caused by malabsorption, remember this is the first deficiency that is showing up, but not necessarily the only one. So while B12 or iron might solve this one facet, if he is struggling to get the nutrients from his food you want to address the large problem as well as the immediate issue. Smaller more frequent meals may also help.
Be sure to ask the internist about Reglan and Pepcid, I don't know enough to comment on the use of these drugs, but the specialist certainly will. I do see online that Reglan is not recommended for long-term use, and is to be used cautiously in kidney failure patients. Since kidney failure is one of the causes of non-regenerative anemia it's worth bearing in mind. Both drugs are working to decrease how much digestion occurs in the stomach (Reglan by shortening how long the food is there, and Pepcid by reducing acid which breakdown food), so it's possible these are making nutrients less available downstream, although I really am not qualified to make these judgements. The internist, on the hand, will definitely know. You may even be able to use these drugs, and see a benefit by giving them at specific times in relation to his meals. Reglan is also used to treat anemia (internet says so), so definitely do not change his medication without a vet's approval.