Unfortunately there is no medicine to ensure that one doesn't take bhang. But what you can do is to let your brother know about the harmful effects of bhang. Many people think that it is harmless and do not know that it can even cause psychosis
. Try to motivate him to quit. There are no major withdrawal symptoms
. Also try to make sure that he does not move around with people who take bhang/ganja. His urine can be tested from time to time for the presence of cannabinoids, to know whether he is abstaining or not.
Hope this helps. Best wishes.