Lots of good responses.
I would also add to give him options - all of which you're okay with. It will help him feel like he has some control, but you'll still get what you want.
For example:
Which veggie would you like with dinner? Peas or Carrots. (it's his choice, but he's still eating veggies)
Would you like to clean your room now, or after dinner? (followed up with "great, I'll remind you after dinner that you chose to clean your room then!" (happily... without sarcasm...))
How many pages of your book are you going to read? (not, are you going to read it or just go read it)
One of my younger brothers was extremely aguementative, that's how my Mom combatted it... and we are still astonished that he didn't grow up to be a defense attourney!
And, a phrase to repeat to him over and over is "I'm on your side". It's a simple way of reminding him that you're not the enemy. There's also a book by that title - it's for teens, but might be worth checking out early.
http://www.amazon.com/Im-Your-Side-Resolving-Conflict/dp/...
I hope this all helps you! Stick with it - nip it in the bud!!
T.