“behold.” The Greek word is idou (#2400 ἰδού), and it is used to get our attention. See commentary on Matthew 1:20.
“he fell on his face.” The man bowed down in a typical oriental fashion, that is, first he got on his knees and then he bowed over with his chest and face to the ground. This action is expressed differently in the three Gospels that contain this record, Matthew, Mark, and Luke, but comparing the Gospels gives us the whole picture. Mark says that the man kneeled, which was the start of the process, then Luke says he “fell” on his face, or “went down upon his face,” which is what the man did with the upper part of his body after going to his knees. Matthew simply records the whole process by saying that the man paid homage to Jesus, which in common biblical manner was to go to one's knees and then put the upper body and face to the earth. Kneeling with the upper body and face to the earth was the common way to act in homage to people and in worship to God (or a god), and it occurs throughout the Old Testament and as early as Genesis (see commentary on 1 Chron. 29:20). That way of showing homage did not cease in the New Testament, and here we see it with this diseased man before Jesus. His kneeling and then bending his body and face to the earth would not have been different than what Abraham did before the Lord some 2,000 years earlier (Gen. 18:2).
[For more on the same word being used for paying respect and worship, see commentaries on Matthew 2:2 and 1 Chronicles 29:20.]