Brief Answer:
Needs investigations.

Detailed Answer:
Hello,
Thanks for posting your query.
I think you mean to say that there is a pain in your father's legs.
In the absence of any history of wear & tear or injury, constant leg pain can be due to:
1. Deficiency of certain nutrients like Iron, Vitamin B 12, Vitamin D, Calcium, etc. which interferes with blood formation & nerve conduction.
2. Decrease in blood sugar levels (Hypoglycemia)
3. Any fluctuation in blood pressure levels such as Hypotension.
4. Hormonal imbalance like Hypothyroidism.
5. Lack of adequate hydration, proper nutrition & good sleep.
6. Even emotional factors like stress/tension can lead to such symptoms.

Investigations like complete blood count, estimation of blood pressure and blood sugar levels will be helpful in clinching the diagnosis.
So, it will be better to consult an orthopaedician & get a proper physical examination done to find out the exact cause.
Based upon that, appropriate management steps could be taken.

Meanwhile, you can try these-

To help the pain, ice the area immediately after getting up in the morning. He can use either a store-bought cold pack or a frozen wet towel. Ice for 10 or 15 minutes. To reduce inflammation, take an aspirin or ibuprofen at mealtime.
A 4-inch wide Crepe bandage wrapped around the region also helps reduce discomfort.
Leg stretching and strengthening addresses the bio mechanical problems and reduces pain.
Use shoes with good arch support and a cushioned sole.
I hope this answers your query.
In case you have additional questions or doubts, you can forward them to me, and I shall be glad to help you out.
Wishing you good health.
Regards.
Dr. Praveen Tayal.
For future query, you can directly approach me through my profile URL http://bit.ly/Dr-Praveen-Tayal

---