Just tell your boss. Its a easy endorsment. Is the co-worker uninformed or just to lazy to go get the tanker endorsement.
Either way not sure why its a big deal...either tell him or the boss. he will get a nasty fine and its his to pay not the company. i had to eat a haz mat fine over a extra oxygen bottle that wasnt covered on a manifest in 2003..was 150 or so then.