What Causes Tremors In The Arms And Head While On Ziprasidone?

My son is 59 years old and has been taking 2 - 80 mg capsules of Ziprasidone and1- 2 mg Benztropine tablet a day since April, 2011. Recently, he has developed shakey arms and head, also has yellow skin around the eyes. He was in a mental hospital for 2 1/2 months in Spring of 2011. He went through a period last year, when he seemed real normal. He did have severe constipation; but is taking 3 colace tablets a day to combat that, and has been since last September. He is non violent and has genuine concern for my health. We walk a mile 3 days a week. Should we make any changes in medication? He does not see his doctor until August. He goes every 6 months. He has always been able to answer most questions on jeopardy; but seem distracted when we watch TV, anymore, even when it s a movie he enjoys