Selfish? No. D., what you're feeling is 'Mom'. It's a big step for our little ones to start kindergarten. Why? Because the years before were just you and the baby. Now it's a milestone in your child's life. He's going to experience a new chapter, learning things, making friends, etc. This is the first step in life.
And you have to let him do it. You know what? You'll survive. I really would not recommend a half week program. Go with what the school offers. It's a transition for you both.
I'm the mom of 3 boys and it was always exciting when they were going to start this. There's not much to be nervous about. Look, I was in K for 1.5 years. Not because I was a flunkie, but because I started in January and at that time they wanted to discontinue having January (high school) graduates. My class was the first not to have them. And I'll tell you I never even NOTICED, HAD A CLUE, whatever that so much time had passed. It was a great time in K.
Let him go, smile through your tears, and realize your little boy won't be little forever. But your backing can turn him into a fine young man.