Does it make you feel any better to know that your DS is VERY normal?
They learn it, use it and sometimes it take awhile for them to use it correctly. It's sort of cute at first but wow, the quickly move on from the no means yes stage... when DS found this stage, I started using this philisophy: Ask questions only
when you are willing to take no for an answer. For all other things (those non-negoatiables that is), make statements. For instance:
If you ask him if he wants corn with his lunch and he says no, respect it (even if you KNOW he really wants it). "ok." If a moment later, he asks for corn, resist the temptation to mini-lecture (you said you didn't want any?!) and just go along, "Oh, I see you've changed your mind." This way he truly learns about what no means and how to use it. He also learns that you respect his right to say it and mean it. This empowers him and helps him to feel that his opinion counts. The more you can do this, the better chance he'll work with you when you need him to...
For those things that are "non-negotiable" try "It's time to get into your carseat now." rather than, "are you ready to get into your carseat now?" If he says no, what do you do? You're knee deep in a power struggle and late for an appointment. You don't want to disregard his honest answer to your question but you don't have a choice other than to miss your appointment. The only thing to do is coerce. You frustrated, late and feeling guilty. He's feeling like his opinion doesn't matter. No fun.
And then there's the time when they learn it, and start using it accurately. "Would you like to go to the park today?" "No", says DS. Mom says, "ok... we'll stay home then."
Lastly, there are those things where you can distract from the 'no' by offering choices. DS has a hard time getting ready for bed--he doesn't want to stop playing. Iinstead of saying 'it's time' and carrying him off crying and fussing, I usually use the "it's time to get ready for bed, would you like to read your plane book or play with your helicopter while we change and dress?" This helps him feel he has power in the situation and he's usually much more willing to work with me then.
Best of luck!
P.S. Edited to say, you didn't ask for advice and I gave you plenty... sorry if I overstepped. It's just that when DS reached this stage, it was disconcerting--he too was such a yes man for so long and I found this advice (found in a book I was reading at the time), so helpful and it empowered us both.