I've learned that if you ask a toddler to do something they automatically say no. It's their way of trying to assert their independence.

I've never asked my child a yes or no question especially if they have to do something. I just tell her to do it.

To help them with their new thinking independence I might ask her things like what she would like for lunch, tuna or cheese? She will get the feeling that she is independent by getting the lunch of her choice, but it would be a lunch that I want her to have.

This way we're both happy.

That's how to do it, just wait it out until it goes away.
It's a phase and usually it doesn't take that long for it to fade. As someone has suggested, try using statements and not questions, it would then give your son no chance at saying "No".