Ianto is frankly a little startled by John's sudden turn, and it takes him a moment to process the fact that he is walking away before he steps forward to lay a hand on the other man's arm.

"Wait," he pleads, because he's really honestly not sure what is going on with him or what it is that he's done wrong here either. "Look, I didn't mean that you -- just wait!" He tightens his hand on the other man's arm, hoping it will help him listen. Will help him stop walking again because he doesn't know what he can have this conversation while on the move, not with his head still aching as it is and John will always have the better stamina than him.