How to clear income source in front of Income tax officer

Dear SIr, My father is a farmer and he is saving some money every year by selling few crops and keep it for the education of my brother. He saved around 5.5 lakhs in 17 years that he deposited in his bank account when he learned about demonetization. He has a pan card attached to bank. Now what further process he should do to clear his income source in front of IT officer or will he be asked any explanation? He is 62 year old man. What should we do now to explain these things? Will the tax officer understand this 17 yrs saving value? My father is not much educated and so he was not believing in saving money in bank. Now its a big headache for me to manage the situation because we have no proof of that income of my father. Kindly suggest any way !