Husband in the ER in severe metabolic acidosis. What does this mean, and what could be the cause?

Many causes. Kidney failure has to be ruled out first. Sepsis, i.e. Infection is important to rule out also. If has diabetes need to make sure it is not diabetic ketoacidosis. Also, important to make sure it is not due to metformin (glucophage) if takes it for diabetes. Other rare causes include Aspirin poisoning and antifreeze poisoning will be ruled out by his doctors.