Imām Ibn al-Jawzī (d. 597 AH / 1201 CE) summarized the scholars' opinions in his book "Zād al-Masīr":

1. It refers to what had been revealed so far of the Qurʾān. [Qatādah, al-Farrāʾ, Muqātil] 
2. It refers to the Prophet (ﷺ), so the meaning would be: "Bring a sūrah from someone like this unlettered servant (of Ours)." [Abū ʿUbaydah, al-Zajjāj, and Ibn al-Qāsim]

What does the pronoun "it/him" in the expression "like it/him" refer to in this verse?

Imām Ibn al-Jawzī (d. 597 AH / 1201 CE) summarized the scholars' opinions in his book "Zād al-Masīr":

1. It means "seek help." (*istiʿānah*)  [Al-Suddī, Al-Farrāʾ]
2. It means "seek urgent aid." (*istighāthah*) [Ibn Qutaybah]

What is meant by "calling upon" their witnesses in this verse?

Imām Ibn al-Jawzī (d. 597 AH / 1201 CE) summarized the scholars' opinions in his book "Zād al-Masīr":

1. They are their false gods. [Ibn ʿAbbās, al-Suddī, Muqātil, al-Farrāʾ] 
Ibn Qutaybah said: They were called witnesses because they witness them and are present with them. Others said: They are called witnesses because they worshipped them so that they would testify for them before Allāh.

2. They are their helpers. [ِAnother narration from Ibn ʿAbbās]
3. It means: "Bring forth people to bear witness that what you produce is like the Qurʾān". [Mujāhid]

What is meant by their "witnesses" who are mentioned in the verse?