The first time I did this fight, I killed only one goblin and lowered the boss's health and then he calling time-out on his own. I reloaded because I wanted the XP from killing the lot, and then knocked out the goblin (using knockout, but he needed to be low health) and triggered the cutscene - but like you, despite having JUST finished interrogating him, Wyll still acts like he died without talking to him first.
