My DS7 is exactly the same. The more he does it the less aggravated he gets. He has sat there for 2 hours staring at a blank page when he had to write a 5 sentence essay.

The more practice the less time he sits there blank.
One thing I read in a writing book that helped a lot was to think about the senses when writing

How did something Look, feel, sound, smell, taste etc
That gave him a tool to use so he isn't blank.

All of a sudden he can write a bunch on sentences to get him started.