Maybe my guess is wrong, but I always figured the usual recipe for teenager creatine trouble looks like this:
-If 20g is good, 40g might be better! (teenager logic)
-I'm invincible! (teenager logic)
-start strenuous training, greatly increasing the body's need for water
-start high protein diet with heavy reliance on protein powder, greatly increasing the body's need for water
-take high loading phase doses of creatine, greatly increasing the body's need for water
The usual consequences of this routine are stomach cramps and diarrhea. But in normal amounts I don't think creatine poses any danger to anyone. Luckily your brother has a wise person guiding him. Just hammer it into his head that a huge dose of creatine is no better than a large dose, and even a medium dose goes mostly unused by the body. And be careful of consuming most of your diet in powder form. Drink enough and there'll probably be no problems.