It seems like your brother is suffering from an infection of his intestines or colitis. This pain can transmit to the back and flanks. They also cause nausea and vomiting. As they are infectious, they are commonly accompanied by fever and headache.
Colitis is inflammation of the inner lining of the colon. As your brother presently does not have diarrhea, it seems like it is a subclinical infection).
I would recommend the following-
1) Start antibiotic for eradication of the organism .Begin with Tab. Ciprofloxacin 500 mg and Tab. Ornidazole 500mg. Stay away from drinking alcohol as it may cause side effects with ornidazole. Ciprofloxacin is sometimes associated with photosensitivity, so it's advisable to use sun screen if you go out.
2) Drink plenty of fluids as you might get dehydrated and it complicates the infection. Mix a sachet of electro powder to a 150 ml of water and drink it in the mornings.
3) Consult a gastroenterologist to schedule a stool microscopy and culture.
4) Eat a good balanced diet with good portion of proteins carbohydrates and vitamins. Consume diet rich in green leafy vegetables and milk.
Consult a physician if the pain does not subside by a week or on appearance of blood in stools.