During The Golem in Honnleath, the Warden meets Matthias in Wilhelm's Cellar. He has activated his father's defences to protect survivors from the darkspawn attacking the village, but his daughter Amalia has gone missing. He will ask the Warden to find her, perhaps in exchange for Shale's correct activation phrase.
Matthias will tell the Warden the correct activation phrase even if Amalia is possessed.
He shows that he can manipulate the magical defenses of his father, implying he may have some knowledge of magic. Whether he is a mage or not is uncertain.