The ideas given so far are the literary equivelant of a sweeping view of a camera, like the opening shot of Oblivion, or the first fifteen minutes of Watchmen. It's a lot of exposition in a very small space. A lot of writers avoid this, or try to incorporate it into a more personal or action oriented section. There's one peice of advice that a lot of modern writers take up. Start with something exciting. A death in a murder mystery, the opening action of a tarantino film, or something like that... a particuar one that worked rather well was in the book "Silver Screen" by Justina Robson. It started with an argument about the nature of artificial intelligence... or something along those lines (i haven't read it in a while.) But basically it sets out the world and themes involved, but does it through an exciting situation (a heated argument) involving the main characters. So you get the main things introduced within a paragraph.
For your story, I personally would start at the execution. Normally they're very public events, held in the arena before the gladiatorial matches, and due to the political machinations behind it, there should be a lot of politicians present. We should see things very close to the son's view. He knows why his father was executed and that it was all politically motivated. He watches these politicians smile over the silencing of an opposing voice. The public would also be present, and they should talk about the supposed crime the father was supposed to have committed, completely believing it when the son knows that it's not true. That his father was killed for his actions against corruption.
It can either be in close third person, or first person. Maybe he can have co-conspirators, with more moderate views to contrast against the son.
But again, this is nothing definite. It's all your choice on how you go about things.