How do you deal with a spouse who doesn't believe you know what your talking about?

Get the doctor to write it out for him in a big, clear way. Then you just point to what the doctor wrote.

I would bet between the diabetes and the stroke, he has cognitive decline and even with a note from the doctor, he may not listen. Good luck!

Does he also have dementia? It might help to read the posts on this site about all of the issues he has in order to see if you can find something helpful. Not sure how long you've been married but I sometimes think this is a "normal" or at least common complaint of longtime spouses ( the part about pretending to not know what you've talked to him about).
Does he have trouble hearing other things? My husband is very good about taking his meds but my aunt has to be reminded daily or she forgets. My mom always put my dads meds in a little dish by his plate at meal time. This made it harder to forget or ignore. My husband does forget to test and resists that. I tell him I'm not cooking carbs until I know what the numbers are. That doesn't always work but its a reminder we are both dealing with his issues and I want to support his efforts to comply. It's very hard to watch someone you love self destruct. Take good care of yourself.