There can be many medical reasons why someone has a low albumin level other than malnutrition, including infection, inflammation, and liver or heart problems. Have other causes of his low albumin been ruled out?
What kind of dialysis does your brother do? The peritoneal dialysis treatment removes more protein than hemodialysis. Does your brother have some kind of GI problem that limits his body’s absorption of nutrients from eating or does he not have much of an appetite? There are drugs that stimulate appetite. Here’s an article about the use of one such drug in people with kidney disease and on dialysis. https://www.mdedge.com/clinicianreviews/article/97389/nephrology/megestrol-acetate-ckd-and-dialysis-patients
Medicare Part B covers “parenteral” and “enteral nutrition” for eligible Medicare patients under certain conditions. You might want to share this CMS info with your brother’s doctor to see if he qualifies for Part B coverage. This is discussed in Section 180.2.
If Part B isn’t an option, the NCOA Center for Benefits Access fact sheet describes what standard Part D plans can and cannot cover. Products for weight gain are excluded by law in standard Medicare Part D plans. However, some “enhanced” Part D plans may cover some nutritional supplements. NCOA fact sheet also has links for pharmaceutical assistance programs.
You can use the Medicare drug plan finder at https://www.medicare.gov/find-a-plan/questions/home.aspx to see if any plans cover what he’s getting. To do this, you’ll need to know the name of the product. You might want to ask the dietitian or doctor what other products might work as well.
Does your brother have Medicaid or VA benefits? Those may cover nutritional supplements. Your brother’s dietitian should be able to help you research options.