Actually he's not. The memo is House Permanent Select Committee on Intelligence work product. The man with the final say so over its release Devin Nunes, Committee chair. He can sit on it or release it with or without the President's approval. However, since a lot of the sourcing for the memo is from classified Administration sources, he wants Trump's approval before he releases it.
They are holding out on the memo based on the classification, saying that it would reveal methods and sources. Trump can declassify the document because he is the top classification authority. They can still choose not to release it and then it still falls on the house to give it up as no longer classified.