You don't say how old your brother is or if he's just getting this in one hand. Does he have any other symptoms. Did it just start up one day all of the sudden or come on slowly. Does he have tingling or other sensations also. In general terms, numbness can be caused by:

1)local injury to the nerves under the skin
2)lack of blood supply to the area
3)pressure on the nerves, caused by a herniated disk, tumors, abscesses or arthritic bones
4)toxic action on nerves (lead, alcohol, tobacco)
5)diabetes and other chemical abnormalities
6)vitamin B-12 deficiency
7)hypothyroidism
8)carpal tunnel syndrome
9)drugs such as chemotherapeutic agents, chloroquine, D-penicillamine, isoniazid, nitrofurantoin, parenteral gold therapy and phenytoin
10)long-term radiation
11)transient ischemic attack (TIA)
12)stroke
13)multiple sclerosis (a less common cause of numbness)

Of course some of these things might not apply depending on if its effecting one or both hands.