Answer By Dr Zakir Naik,That depends, if as long as he does not do any Haraam act during the fast. For example, It's mentioned in the Hadith of Muhammad (saw) in Sahih Muslim, Vol. No. 2, (Book of Fasting), Hadith No. 2453, it's mentioned that, :Umme Salamah (may Allah be pleased with her) who was the wife of the Prophet she says, that the Prophet got up from his sleep in a state of sexual defilement, state of sexual impurity while he was fasting and he had a bath and he continued his fast."
Further there's a similar message given in Sahih Bukhari, Vol. No. 3, in the Book of Fasting, Hadith No. 1925, and the Hadith is narrated again by Umme Salamah (may Allah be pleased with her) and Hadhrat Aisha (may Allah be pleased with her too) both were the wives of the Prophet and they said that"The Prophet used to get up in the morning in a state of sexual defilement, because he had a intercourse with the wife, not because of a sexual dream and he used to have a bath and he never used to compensate the fast later on."This proves that being in the state of Junub, sexual defilement or sexual impurity does not nullify the fast. Even if a person has a wet dream in between, his fast is valid, the only point to be noted is that the person should have a bath as soon as possible. If he gets up in the morning after dawn breaks and if he's in the state of sexual impurity, should have bath immediately so that he can offer the Salaah of Fajr. Or if he sleeps after Fajr Salaah and has a wet dream and then becomes sexually impure so he should have a bath atleast before the Zuhr Salaah so that he can offer the Zuhr Salaah after having the Ghusl.