After my dads stroke, his dr put him on Prozac and Remeron (mirtazapine) to help him sleep! Can these 2 drugs together cause severe mood changes? What do I do?

Yes. I would not use either drug with a patient who has had a stroke, as the adverse effect profile might present in a far more intense fashion, especially suicide risks with Prosac. I personally feel uncomfortable with the approach, and recommend a visit to a geriatrician or psychiatrist to get this straightened out.