Your father's experiences with sleep problems after a stroke are common. Fortunately, in most cases, it does get better. Your neurologist can give you a lot more details about all the different options for treatment.
One issue that is not addressed often enough is to screen for obstructive sleep apnea in all stroke patients. Studies have shown that the vast majority of stroke patients have undiagnosed obstructive sleep apnea, which is a major risk factor for stroke. If he has obstructive sleep apnea, then that's another good reason for his sleep problems.