A:
There is no doubt that Zina is major act of disobedience and an abominable sin. Allah (Exalted be He) says:
And come not near to unlawful sex. Verily, it is a Fâhishah (i.e. anything that transgresses its limits: a great sin), and an evil way (that leads one to Hell unless Allâh forgives him).
Therefore, it is Wajib (obligatory) on a Muslim to avoid Zina and all ways leading to it. Whoever practices Zina must hastily offer sincere Tawbah (repentance to Allah), perform good deeds, and protect their private parts from all that Allah has forbidden. As for what is mentioned by the questioner
that he remained Junub (person in a state of major ritual impurity) until he had Suhur and began Sawm, his Sawm is valid, but it is impermissible for him to abandon the Fajr Prayer and delay the Janabah Ghusl (Part No. 9; Page No. 176) until noon. It was Wajib on him to hastily perform Ghusl and offer the Fajr Prayer along with Tawbah from Zina and determining not to commit this sin again.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.