Reach 75% like with Little Jacob, friendship is unlocked after the mission 'Shadow' is completed.
Little Jacob likes the following activities:
-Eating *Preference- Cluckin' Bell
(Only call between 1pm-4am or % will go down for waking him up)
Friendship goes down in % by:
-Late to meeting (minor impact)
-Don't show up (moderate impact)
-Call Little Jacob and he will drive to a location near you and you can buy weapons from the trunk of his car.
When completing one of Jacob's jobs (on the phone, NOT main missions) in single player, his respect for you goes up 1%-3% every time, so it may be worth doing this achievement once you've completed the campaign.
*Note* If you vote down please PM so i can improve my solution
*Thanks to Vor Calv 06 for improving my solution*