Dear H.,
Now, now, now, you are not going to keep him out of Kindergarten because he yells at home - Kindergarten is an entirely different environment and the teacher will deal with it. She or he may work with you so that both home and school have the same rules about voice level.
Stop trying so hard. My gr granddaughter is loud too and she is about the same age as your boy. Maybe it is a 4 year old thing. Children are not programable - give up for a while, just enforcing the loud for outside is very good. Your family can work with it and my family will work with it, and some day they will learn to speak more quietly.
I used to work with a preschool teacher whose voice was very soft and the children learned to listen more closely - she paid a lot of quality attention to them and they wanted to hear what she said. I really did see it work. When we talk as loudly as they do, then we are demonstrating yelling to them, so that is not good. Develop some signals between the two of you that mean quietness.
Good Luck, C. N.