Can my father, who is 58 years old and did not pray or fast, atone for past years after his repentance to avoid being held accountable?

Your father has committed grave major sins by abandoning prayer and fasting, and abandoning prayer is considered disbelief by some scholars. He must repent immediately and observe prayers and fasting, and make up for the missed days of fasting. As for the missed prayers, there is a difference of opinion regarding the obligation to make them up, and it is better to be cautious and make them up. He should also increase in supererogatory acts of worship (Nawafil), supplication (Du'a), adhere steadfastly to Allah's law, and associate with the righteous.