OP: It would probably be a combination of things, like blood loss, electrocution, & shock to weaken him, then restricting his breathing while multiple men come in his throat at once (no doubt wearing a spider gag or similar to hold his mouth open).

Maybe like someone pinches his nose shut & holds while they're coming to try to force him to swallow and they accidentally drown him when he aspirates a couple loads of semen at once & can't cough it back up, for example.