I don't know how long your husband took to get to such a high dose, but it probably didn't happen overnight. A dose that would probably kill a non addict doesn't kill the addict because he or she has developed a tolerance to the drug.
The trouble with tolerance is that it gives the false impression to the addict that they don't risk overdose and death. Anything that potentiates the drug, like using alcohol and opiates, or benzos and opiates, or other combinations, can prove lethal to someone taking their "usual" dose. Sometimes even their usual dose is enough to become a toxic overdose if something shifts the balance in that direction. You hear about accidental overdoses all the time.
I'm not telling you this to scare you, but he's probably built a "confidence" or at least a lack of fear that he can't kill himself on those doses. He may be wrong. I'd strongly encourage you to talk to a doctor and ask these questions. I doubt those doses are medically prescribed. There are drug hotlines in most big cities where you can ask questions anonymously and get the answers you seek.