My female budgie has layed eggs but male doesn't seem to go inside nestbox but sits at step of nestbox?

She will let him take his turns eventually. She has to make sure he understands the principles of nest sitting.
Well anyway, I have observed the same thing: Both are inexperienced, I assume, and she is sitting the nest because she laid the eggs and has the nesting instinct to start with. But he will observe and when she decides she needs a break, he will sit the eggs for a while. Then they will take turns, and raise the chicks together.