Although B-12 is good. I have learned it really does not much good unless you are defistient. I would consult his doctor about what diet is best for him and proteins etc. There is alot more out there for athletes.
Whey Protein is supposed to be very good. Put some weight on him. My son use to take that and mix it in food or a drink.

this is what i found for B12
Vitamin B12 (Cyanocobalamin) is a water soluble vitamin necessary for energy production; for nervous system function; for the production of acetylcholine, a neurotransmitter that helps with memory and learning; for the synthesis of red blood cells; and for producing the genetic materials, DNA and RNA

if you want Whey protein, you can try HMS90 Immunocal. =)
I been taking them for years as well.

You body naturally produces B-12. That a doctor will usually do is run a B-12 deffincieny test. If you are low on B-12, He may recomment a monthly injection for you. Honestly. If you are not B-12 deffiffient, you are wasting you money and your time by taking it. Now, in a multi-Vit that is fine. The dose is a small in % of daily requirment that it really has little effect. B-12 in measured with a blood test.
I would still go with the Whey Protien IMHO.
Carrie