All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
This man is only obliged to perform Ghusl (ritual bath) once, however, it is desirable for him to perform ablution after each sexual intercourse so that he will be more active. Anas narrated that the Prophet used to go around [has sexual intercourse with] all his wives with one Ghusl (ritual bath) only. [Muslim]
Allaah Knows best.