All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
If he had intercourse with you during a voluntary fast, then there is no problem, but if it was during an obligatory fast, then you made a mistake in enabling him to do that, even assuming that he forgot he was fasting. However, you should have reminded him and prevented him from that, because religion is advice. Moreover, if this happened in Ramadan, then there is nothing on you because you were not fasting that day, provided that you broke your fast for an excuse such as sickness or your menstruation stopped during the day, so you do not have to abstain for the rest of your day according to many scholars. However, if you broke your fast without an excuse, then you have sinned by enabling him to have intercourse, and you must make it up, and there is a difference of opinion among the scholars regarding the obligation of expiation. The opinion that we adopt here at Islamweb is that the woman is not obliged to expiate, which is the view of the Shafi'i. As for your husband, if he did what he did out of forgetfulness, then there is also a difference of opinion among the scholars whether he must pay expiation or not; the Hanbali School made it obligatory, but the majority of the scholars did not make it obligatory. Nevertheless, the view of the majority of scholars, which is that he does not have to offer expiation, is the view that we adopt here at Islamweb. As for Ghusl, it is obligatory for you both before the time for prayer ends, so that you do not delay your prayer.
Allah knows best.