In the name of Allah, the Most Gracious, the Most Merciful. What is the ruling on a child washing his deceased father?

If the intent of washing is for a son to wash his deceased father, then there is no harm in it. The father is more entitled to wash his son if he is able to do so without harm and knows the method of washing. If the intent is for him to assist his father in ghusl (ritual bath) during his lifetime, then there is also no harm in it, with the necessity of avoiding seeing or touching the 'awrah (private parts) except out of necessity.