Lure him into your castle (there should be a hero stationed on top of the garrison) so that he's there during your turn, and then enter the castle and press the spacebar. After that, the hero will supposedly "consolidate," and you'll be able to control him, but most likely, his mind will glitch, and he'll just wander back and forth during the ally's turn, and during your turn, he'll just sit there with no movement points.