Your father needs to have a complete neurologic workup that includes bloodwork, MRI of the brain, MRA of the vessels of the head and neck, carotid artery ultrasound
and possibly even an echocardiogram
. This is the standard testing done for stroke
and if it hasn't been done in some time, it should certainly be repeated. Consulting with a good neurologist
whose practice centers around stroke is a must, because the neurologist will help your father to control stroke risk factors that will help him to prevent another such event from happening. I hope this information has helped you and that he feels better.