Even if General Hux is not the son of Luke Skywalker, he could still be working undercover as a Resistance spy. This simpler, stripped down theory is more plausible since it ditches all the convoluted Skywalker/Jedi backstory. The film would no longer have to get into the territory of "Who is his mother?" and whether Hux has Force abilities. Best of all, making the General a Resistance agent has the advantage of showing the heroes as proactive and capable of the types of intelligence operations movie goers would expect from a competent military. It also provides a way for the Resistance to find out about the First Order's plans so they can get on with stopping them. On top of that, assuming that the General comes to a sticky end when the First Order discovers his true allegiance, Hux's ultimate sacrifice could spur the good guys onto even greater acts of heroism. Really, all that's required for audiences to swallow this scenario is for the General to have some kind of emotional link with the Resistance. And that link could be supplied easily enough by saying that Hux is friends with another character, like Oscar Isaac's X-Wing pilot Poe Dameron.