I say, and with God's success: It is not obligatory for him to perform ghusl (ritual washing) unless he has ejaculated; because the cause for ghusl is ejaculation, and penetration is considered equivalent to it, as it is often a cause leading to it. This causation is only realized when desire is fully developed, and here it is incomplete, and it is neither explicitly stated nor in the meaning of what is explicitly stated. See: Al-Wiqayah p. 95, Fath Bab Al-Ianayah p. 321, and Al-Sayah p. 321, and God knows best.