His body has a chance to spawn at right bottom of the Hive Cluster level. You can unlock Dead Man's Foot by pressing 'Use Item' button over it. It will explode, unlocking said item, and also spawn a single Archer Bug.
Note: Doesn't appear in online games. Sometimes there's a teleport instead of the body. In case there is no Bloated Survivor, you don't have to start over — the body is likely to appear when you re-visit the level.
Poor guy. It looked like he was infested by some bug-like creatures, and exploded when I got close. I hope his death wasn't too painful; his family will know how he died. Rest in peace.