A son can cut his father's hair, especially when his father is ill in bed. A son has the obligation to honor his parents, including cutting his father's hair.
It's really normal for my son to cut his father's hair and wash his hair. As a son, my father is getting older every day, and it will be inconvenient to move, especially to wash his hair and need help from others. Therefore, as a son, he should wash his father's hair or help him get a haircut, which is a sign of filial piety.