Risperdal is an antipsychotic given for many disorders at this point although it was originally intended for schizophrenia. There are other medications that are traditionally used for bipolar. I think you really need to consult a pediatric psychiatrist for decisions revolving around medication----- and there is also a doctor's forum here that may be of more assistence. But that is all so personal to the child ----- only a professional in your area can give you legitimate advice. Good luck.
What symptoms does your son manifest?