400mg is usually the starting dose of this medicine. The dosage of Tegretol (Carbamazepine) is not made based on how many mgs he takes a day but in the plasma levels of the carbamazepine. The dr. might request a blood exam next time your brother goes there. The drug is usually taken in the morning and then in the night. I also take the same medicine but for other reasons.
While you are taking the drug you need to make some exams regularly in order to avoid the more serious side effects. I read that it is necessary to make assessment of liver function tests, complete blood count, reticulocyte count but don´t know exactly what those mean.
Also if your brother is taking any other psychiatric medication Tegretol will lower its efficacy in the beginning of the treatment but later it will return to normal.
I found something about agitation too. Reading "The Medical Basis of Psychiatry" by S. Hossein Fatemi, MD, PhD: "Other potential, side effects include drowsiness, loss of coordination, vertigo, ..., impaired performance in learning and memory tasks, irritability, agitation, insomnia, and emotional lability". But you need to know that just because there is a potential side-effect does not mean that it will necessarily happen.
Good luck.