Does my father need to clean the alleged place where he was licked by a dog a few decades ago from najis in order to begin doing prayers?

My father is a Muslim, but unlike me, he does not do prayers. In his youth, a few decades ago, he was licked by a dog. As far as I know, the dog belongs to a hard-to-remove najis, and to remove it, you need to wash the place 7 times with water, and 1 time with water mixed with clean earth so that the water will be cloudy. The question is, does my father need first of all to clean the alleged place where the dog licked him from najis (my father does not remember exactly where the dog licked him), since he wants to start doing prayers?