Can a son donate blood directly to his father?

Generally speaking, it is not allowed.

Because of the high blood similarity between immediate family members and the low resistance of patients, graft-versus-host disease may occur during blood transfusion between immediate family members. Graft-versus-host disease (GVHD) is an immune response, which can cause severe organ damage in recipients. Therefore, in order to avoid risks, sons should not donate blood directly to their fathers. It is suggested to find other suitable donors or choose other treatment methods.