Jackson Whittemore has been Theo's best friend even before they started to attend Hogwarts. Jackson is the only one who knows of his bad relationship with his parents, but Jackson keeps his mouth about it to others because it's not his business to tell. Based on the stories Theo told him, Jackson refuses to ever go to the Raeken household, which is not going to happen at all because Theo doesn't invite anyone to his home. Instead, Theo either always stays at Hogwarts during the holidays or he stays somewhere else (this place is now the Hudbar household ever since his third year.)
Theo and Jackson are the "Head Snake" of Slytherin with Santana acting as their "Deputy Snake", their second-in-command (which caused a lot of anger and irritation with Alex Russo, especially now that Santana is teasing her about her status while she has none.) Theo and Jackson had to fight hard to gain this status within their House as many students from higher years protested against it or thought they could snatch that title from them but in the end, the duo got what they wanted and won.
Theo and Jackson decide what happens in Slytherin House and what the rules are that every Slytherin has to follow. Whatever they say is law. Their Head of House, Professor Snape, knows about what happens within his House as is, in fact, encouraging it. Professor Snape hopes that this will bring some structure back into his House.