Does he attend pre-school or school? what's his behaviour like there?
Have you ruled out ADHD - is it worth getting him checked ?
Some of it could be of course that he is the youngest of five and he is really, really having to fight for his position in the family, for enough attention, for the toys he wants, for time alone....lots of things might feel like a real fight to him because of his position in the family.......#i'm guessing here, am mum of one!! but this sounds like it is common sense really.
If there is no 'medical' side to this behaviour then I think you just need to deal with his behaviours in a very clear and consistent and no-nonsense way, for example running across roads - don't give him the chance. Unless he can hold your hand sensibly, he has reins on....time out step I think is a waste of time, just asking for it to become a game - if he is really being impossible or has hit someone or something serious, I think 4 minutes alone in his room is far better; genuine time out for everyone to calm down, not as a 'punishment'.
You might need to accept that you're both in a downward spiral at the moment, your negative reactions (while completely understandable) may just be worsening the situation as he is more negative in response. Always make sure that you move on from things, don't stew on them or bring them up after the event, forget about past behaviour and concentrate on the here and now.
What do you think, have you had him assessed for anything or is this just extra challenging behaviour?