Causes of insomnia or sleeplessness are numerous. It could vary from a normal age related change to substance use related change to a diagnosis of depression/anxiety/functional insomnia. Your father needs to be evaluated in detail by a Psychiatrist and if need be, he will be referred to a Neurologist. Although medications are used in the treatment of insomnia, assuming that everything would be set right by pills alone would be an oversimplification.
Next Steps
Consult a Psychiatrist in your vicinity and on a regular basis.
Health Tips
Try and learn about 'sleep hygiene' from online sources which are reliable and explain it to your father.