How is it you voice still scares your own child? My voice is two steps uhigher than Barry Whites and my kids have never been afraid of mine. In fact it's kind of funny how many kids try to imitate it.
Otherwise Cathy has solid advice.
I still have this problem at the age of 35, in school I could never concentrate especially when I was in a class where I hated the subject. I am a teacher and I would strongly suggest talking it over with the teacher/principal for ideas. Find out what is strengths are first what subjects he really enjoys and reward him by saying that the two of you can work on something that he really enjoys if he just concentrates for awhile first. Maybe he just really doesn't understand what is being taught! You can buy some extra activity sheets on the subject or download off of the computer on teaching websites. Setting goals and having a calendar in the house you can write his assignments on might help too! I hope these suggestions will help you, if you can't handle it I know they also have learning centres (depending on where you live like Sylvan- they will help your son with homework and assignments). P.s. I could never sit with my parents or get them to help me, it is hard for him because he probably doesn't want to disappoint you!