Everybody has a different learning style, and even within the same group types individuals all take a slightly different approach to things. Teachers should
be properly trained to spot each student's style and lead the class at a pace which suits everybody. However, the number of teachers actually capable of this is small.
First of all, I would recommend identifying your learning style, if you don't know it already. By finding out how you learn best, you will also be able to clearly see why you aren't doing well in this particular teacher's lessons. This website
has a test which you can quickly take to find out which learning style you fit into.
After you know how and why you learn the way you do, approach him after a lesson or during a break and calmly say that you are having trouble understanding things, and which parts of the lesson you're having trouble with. At this point it would be helpful to him for you to mention your learning style, and why you're hesitant in approaching him. Chances are he's a big ball of stress, so you telling him that you're afraid of being shouted at for not understanding may have more of an impact than you might think.
If this doesn't work, then you can always go and talk to the school counsellor or even the principal. When you're explaining the problem, don't be too tempted to pin all the blame on the teacher. Just say why you're having trouble in his classes and with which specific parts. Even if they can't do anything about him in class, you will at least be in a better place when it comes to studying and revision, knowing exactly which parts you need to focus on.
I hope that helps DX