Little Jacob is unlocked after completing the mission from Roman, Jamaican Heat.
In this mission, you and Little Jacob are going to go and settle the score for the ambush earlier. Get in a car and drive to the house marked on your GPS.
This is an easy mission to train you in taking cover.
When you get to the front door, take cover, and Jacob will kick the door open. Gun down the thug that comes down the stairs, and then move to the window to take out the enemy holding a shotgun. Go inside and take out the remaining enemies, and drive Jacob home to complete the mission.
Little Jacob and Badman have asked Niko to take care of some rival dealers on their turf. Go to the spot marked on your GPS and watch the short cutscene. Keep a good distance behind the dealer, and keep following. At one point, Roman will call. This is not an issue if you're a good distance away from the dealer though. Follow him into the apartment, and take him and his cohorts out to complete the mission.
After completing this mission you will unlock Little Jacob's job option.
If you call up Little Jacob and select the job option, you will be pointed in the direction of a car to pick up with drugs inside. When you get in the car, you will have a limited amount of time to drive and park it at a specific location. If you fail this mission, you will lose reputation with Little Jacob.