What are the differences between moderate cognitive impairment and severe cognitive impairment?

I have generally found the charts in this article to be helpful; I hope they are useful in framing questions for your husband's doctor.

https://www.dementiacarecentral.com/aboutdementia/facts/stages/

If your husband is self-aware enough to know that his skills are somewhat worse, it doesn't sound to me (a non-professional) as though he is in the "severe" category yet. As SJ notes, please make sure that treatable causes like non-pressure hydrocephalus and tumor have been ruled out.

Different dementias progress differently. While generally, you can't determine the type with certainty until after death, there are often clues that tell doctors if it is Alzheimer's, Lewy Body Dementia, Vascular Dementia or "mixed etiology".

Moderate or severe cognitive decline doesn't say much. How is he cognitive impaired? What is the cause of his "cognitive impairment"? When he goes in for his next evaluation, make certain the doctor states the cause. IF he says dementia, that is not sufficient. Dementia refers to symptoms and is not a disease. Dementia is usually caused by a neurodegenerative disease like Alzheimer's or Lewy body. Different diseases progress differently, manifest behaviors differently and are treated (although temporarily) differently. Only when the cause is determined will you know what your dealing with. There are many reasons for cognitive decline and dementia. Make sure he's told the reason for his decline.