After a disagreement, he left to join the Forged, a group of fire-worshipping fanatics established in the nearby Saugus Ironworks. When he left he took his father's Shishkebab, a family heirloom, to give to the Forged. His hope was that it would help gain favor with their leader Slag and that they would raid settlements outside the Commonwealth.
PCPlaystation 4Xbox One If you rescue Jake, sometimes he will stand in one spot when he returns to the farm. He will not work on resources and you cannot trade with him. A potential fix is to assign him to a supply line and let the bug work itself out. [verified]
It is caused by not having joined the Minutemen. Its impossible to talk to Preston Garvey and say to him that you have rescued Jake Finch, since he is stuck on his request to join the Minutemen. If you reply "yes" to Preston request, after that, waiting a while, Preston will talk to you saying that he is happy you saved Jake. Going back to Finch Farm, you will find Jake unstuck, and available for trades/assignments.
PC Player may be unable to relieve Jake Finch of supply line duty. He can't be moved to another settlement and refuses to take on work assignment. [verified]
One way to fix it is to stealth kill Jake away from his farm, so the Finch family remains friendly towards the player.