I tried to persuade him to change his mind, but he remained adamant.

Foolish : lacking good sense or judgement; unwise.
Aggressive : ready or likely to attack or confront; characterized by or resulting from aggression.
Stubborn : having or showing dogged determination not to change one's attitude or position on something, especially in spite of good arguments or reasons to do so.
Defensive : used or intended to defend or protect.
Adamant : refusing to be persuaded or to change one's mind.

Synonym of Adamant is Stubborn