I say, and with God's success: He is not required 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. See: Al-Wiqayah p. 95, Fath Bab Al-'Inayah p. 321, and Al-Sayah p. 321, and God knows best.