Primarily, what draws Jennings to Sal is that he's like the father he never had. Growing up without a father figure has had a serous impact on Jennings' life, so much so that he hasn't been able to develop properly as a young man. (He learns more about fatherhood living at the Bronx Zoo than he does at his various foster homes.)
Sal takes in Jennings after his mother's injured, providing the boy with the stable home life that he's been lacking up until now. For the first time in his short life, Jennings has found somewhere he can call home—somewhere he can feel safe, loved, and appreciated. He instinctively responds to Sal's friendly, caring nature, seeing in him the father he never had but dearly wishes he could have had.