Extrapyramidal movements, although well known, are a rare side effect of the antipsychotic medications. Doyle624, if you are concerned that your son's twitches are caused by the Risperdal, do this. Have him open his mouth and observe his tongue. Usually, the first extrapyramidal movement to appear is an involuntary "snakelike" movement of the tongue.
Tics are common in kids. Tics can be brought on or worsened by stimulants like Concerta, though. If that's the case, they normally disappear when the stimulant is stopped. Unfortunately, they don't always.