There were two reasons Austin wanted to meet that man at the eatery even though he was aware of the demon's presence in the Academy.
First, he wanted to assess the credibility of the man, and second, he wanted Valerie to understand the origin of his information. If he had simply told her about the demonic entities, she likely wouldn't have questioned his source. However, he valued transparency and chose not to conceal anything from her, openly revealing where his knowledge came from.
In the future, if she needs to gain some information, Austin has told her to get the passcode from either him or Sebastian; since it changes daily.
Now, back in his room, Austin was sitting on his bed, and studying his dagger.
It was a sixteen-inch-long dagger, which appears without any details. After all, his Shard has yet to receive its first evolution.