John Gill Commentary Luke 18:38

"And he cried, saying, Jesus, thou son of David, have mercy on me." — Luke 18:38 (ASV)

And he cried, saying, Jesus you son of David
Believing him, at once, to be the Messiah; from which 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.