I would take him to the dr and just check it out, it could be something like scabies that he's picked up from somewhere that has got nothing to do with the bed at all? The dr might be able to I.D, the bites based on the pattern.

Also worth checking what's happening under the floorboards, I have heard once of a nasty infestation of some sort of tiny biting licey thing in a classroom, apparently they didn't get it sorted out until after they treated underneath the floors.