Is not this the Son of David? - That is, Is not this the promised "descendant" of David, the Messiah? They were acquainted with the prophecy in Isaiah 35:5, "Then the eyes of the blind shall be opened, and the ears of the deaf shall be unstopped," and they inferred that he must be the promised Messiah who was able to do this. This inference was drawn by the common people, and not by the proud and haughty Pharisees. It is not uncommon that people of plain common sense, though unlearned, see the true meaning of the Bible, while those who are filled with pride and science, falsely so called, are blinded.

Is not this the son of David? - Is not this the true Messiah? Do not these miracles sufficiently prove it? See Isaiah 35:5.

And all the people were amazed,.... At the cure; it was such an instance of divine power, and so glaring a proof, that the person who wrought it was more than a man, and must be the Messiah. This is to be understood of the greater part of the people, not of every individual, and of the common people only; for it had a different effect upon the Pharisees, as hereafter appears; but in these it not only produced admiration, but conviction, faith, and confession:
and said, is not this the son of David? or the Messiah; for , "the son of David", is a character of the Messiah, well known among the Jews; See Gill on Matthew 1:1 because he was promised to David, was to be raised up of his seed, and to spring from his loins. This question they put, not as doubting of it, but as inclining, at least, to believe it, if not as expressing their certainty of it: and is, as if they had said, who can this person be but the true Messiah, that has wrought such a miracle as this? for from his miracles they rightly concluded who he was; though the Jews since, in order to deprive Jesus of this true characteristic of the Messiah, deny that miracles are to be performed by him (n).
(n) Maimon. Hilch. Melacim, c. 11. sect. 3.

And all the people were amazed, and said, Is not this the son of David?--The form of the interrogative requires this to be rendered, "Is this the Son of David?" And as questions put in this form (in Greek) suppose doubt, and expect rather a negative answer, the meaning is, "Can it possibly be?"--the people thus indicating their secret impression that this must be He; yet saving themselves from the wrath of the ecclesiastics, which a direct assertion of it would have brought upon them. (On a similar question, see on John 4:29; and on the phrase, "Son of David," see on Matthew 9:27).

Is not this the Son of David? The promised Messiah King who was to be the Son of David, according to the prophets (Isaiah 11:10; 2-Samuel 23:5).

Is not this the son of David - That is, the Messiah.