I've fainted since I was 9; but didn't have a doctor take me seriously until I was 42.
Please mention a tilt table test to your parents or the dr to check and see if it's heart or ANS (Autonomic nervous system) related; plus HCM/HOCM as grendslori said should be ruled out.
Fainting can be caused by a vast number of things from things like dehydration, low blood pressure, infection to HOCM like I was diagnosed with.
Your brother should certainly be evaluated by a pediatric cardiologist. He could have an arrhythmia problem or even HCM which is more common than HOCM which is the obstructive type of HCM. It could also be a vaso response that children and teens sometimes go through which is totally harmless. Have him evaluated.
I know a woman with a young son who occasionally passes out. They said his was a drop in BP. Not sure what caused it but he's healthy otherwise. He has no heart problems. Has your brother seen a doctor for this? That's the first step.