Luke 18:38

And he cried, saying, Jesus thou son of David
Believing him, at once, to be the Messiah; whence he calls him the son of David, which was a character and title of the Messiah, well known to the Jews; (See Gill on Matthew 1:1) and therefore immediately called out unto him, being willing to take that opportunity as he passed, and whilst he was within hearing, to make his suit to him for his sight:

saying, have mercy on me:
a poor, blind, and miserable creature, and restore me to my sight, which will be an act of singular mercy, and goodness, and will always be gratefully owned as such.