My Husband Lost His Entire Stomach To Cancer In 2009.

My husband lost his entire stomach to cancer in 2009. Since then he has taken monthly shots of B12 but is showing signs of brain atrophy. I have read that insufficient B12 can cause this brain atrophy and a pull back in cognitive thinking. How much B12 should he be getting each month. What is the maximum he can get at age 78 to be safe and retain his thinking?