I think that while it's true that a lot of Huck's prejudice towards Jim seems to come from the society he has been raised in, it's also true that he generally forms his own decisions and has no problems defying social norms such as formal education and religion. In spite of this, he personally sees Jim as inferior. He doesn't do this completely mindlessly, but actually makes several statements about Jim's quality as a servant based on his own judgment. For example, after Huck and Tom sneak past Jim and take his hat the night they start their gang, and Jim superstitiously interprets their prank to think that he was flown around the country by witches in his sleep, Huck remarks that  "Jim was most ruined for a servant, because he got stuck up on account of having seen the devil and been rode by witches." Thus, Huck is observant of the way Jim acts and is treated, yet he still seems to take Jim's status for granted. When the two of them meet on the island later on, Huck almost immediately orders Jim to start a fire and help him make something to eat. Yet despite this, he also immediately agrees to keep Jim's secret of running away and listens to Jim's input to some extent when trying to decide how they can best hide and survive on the island. I think the fact that Huck can look objectively at Jim's status and in some ways treat him as an equal, yet still take the fact that he has the right to order Jim around for granted shows how ingrained slavery is in the society Huck has grown up with. Because of this, I think the only way Jim can stay free is to flee to one of the free states such as Illinois and make a permanent break with everyone in his old life. The culture of slavery is too ingrained even a fairly independent-thinking person such as Huck, for Jim to have an opportunity of keeping his freedom while returning to the area he lived in before.