Is ghusl obligatory for someone who had intercourse with his wife before traveling and forgot the ruling, so he penetrated halfway and ejaculate?

If a man has intercourse with his wife during the daytime in Ramadan, he must repent, seek forgiveness, and offer expiation for intercourse. The expiation is the freeing of a believing slave. If he cannot find one, then he must fast for two consecutive months. If he is unable to do that, then he must feed sixty needy people. This order is obligatory according to most scholars, while the Malikis hold that it is a choice.