yellowing of teeth most often happens due to accumulation of deposits of plaque and calculus
, which then collect stains from your food, and from products of bacterial metabolism. My suggestion would be for your brother to see a dentist
. He would benefit from a professional cleaning of the teeth to get rid of the deposits. Also, the dentist would then discuss oral hygiene home maintenance with him, which should help maintain better and cleaner teeth subsequently. The importance of regular dental follow up, in this context, cannot be overemphasized. If he desires further lightening of the teeth after the professional cleaning, there are options available, including bleaching
, veeers, crowns, etc.