I'd be really cautious about using supplements at this point, especially if his doctor is against it. ATG with Cyclosporine has a long track record for the successful treatment of Aplastic Anemia.
Cyclosporine is a very complex drug and there are many unknowns as to why and how it works. The absorption profile varies for different individuals. You really don't want to risk interfering with its absorption and the possibility that your brother may be a responder! Your brother is very early in his treatment, and patience is key at this point.
Dr. Paquette is an expert in this field. Your brother is in good hands.
53 yo female, dx 9/08, AA/hypo-MDS, subclinical PNH, ATG/CsA 12/08, partial response. small trisomy 6 clone, low-dose cyclosporine dependent