Hi.my son is 15 years old boy. but he is masturbating. can i give permission to it as father?

Yes: Since it is considered a normal part of teen development in both genders across all cultures this is fine. It is reasonable to provide limits like "private only" time & duration that never interferes with their other chores or productive activities. Many cultures promote shaming activities which only make impressionable teens self conscious or ashamed of normal sexual feelings.