Dann steht er auf.

Oooh here I opened a can of worms... aufstehen is a combination of two words glued together. Germans like glueing words together. We even have special word glueing games. But I leave that for later :D

What is glued however can also be 'unglued ' again....

Let's look at our example

aufstehen = auf (up) + stehen (to stand).

Together it means: to get up / to stand up

if we use it together with a second verb, nothing bad happens

I want to aufstehen

I have to aufstehen

I dream of aufstehen

But -here comes the tricky part- when we use aufstehen alone, it splits up and you ll never know where the different bits go in the sentence (I know of course :D)

I get up - Ich stehe auf.

I get up in the morning: Morgens stehe ich auf. or Ich stehe morgens auf.