Praise be to Allah.
He has to complete it as Zuhr, because the Jumu’ah prayer is over. You can catch it up with one rak’ah if you catch up with the second rak’ah prayed with the imaam, then you can pray it as Jumu’ah. But if you do not come until after the salaam or after the second rak’ah, during the Tashahhud or when the imaam is prostrating in the second rak’ah, then you should not pray it as Jumu’ah, but rather as Zuhr, because the Prophet (peace and blessings of Allaah be upon him) said: “Whoever catches up with one rak’ah of Jumu’ah, let him add one more to it and his prayer will be complete.” (Narrated by al-Tirmidhi, 482; al-Nasaa’i, 554; Ibn Maajah, 1113). The meaning of this hadeeth is that if he only catches up with less than a rak’ah then he has not caught up with Jumu’ah so he has to pray it as Zuhr. This is what is prescribed in Islam. If he catches up with a person who is making up the rak’ah that he missed, and prays with him, then he should pray it as Zuhr and not as Jumu’ah. It should also be noted that this should be after the sun has passed its zenith. But if Jumu’ah prayer was offered before the sun reached its zenith, he should not pray Zuhr until after the sun has passed its zenith, because it is permissible to pray Jumu’ah before the sun reaches its zenith, in the “sixth hour”, according to the correct scholarly view. But it is preferable and more on the safe side to pray it after the sun has passed its zenith, as is the view of the majority of scholars. But in the case of Zuhr, it is not permissible to pray it until after the sun has passed its zenith, according to the consensus of the Muslims. And Allaah is the Source of Strength.