It sounds like you don't quite understand the concept of time out. Time out is not a punishment. Time out is supposed to be used as a way for both you and your child to separate yourselves from a situation that is escalating, and to give you both a chance to calm down. It has nothing to do with 'think about what you did' or 'this is your punishment for what you did'.
Your child is 2. That means by the end of his diaper change and time out, he probably didn't even remember why he was in 'time out'. So, if you can't stick him in time out as a punishment, what do you do? Well, which problem are you wanting to address? Him telling you 'no' repeatedly, or him not listening to you? Or both? One thing that works really well is to use the consequences that come naturally and logically from his actions. If he doesn't want his diaper changed, then leave him in his poopy diaper for a while. Try it again in a few minutes.
Time outs don't teach appropriate behavior, nor do they actually serve as a good discipline technique.
Edit: I understand that you want to teach your son, but realize that even though he is getting older, he is still learning what is right by watching how you behave. So, if he hears 'no' all the time, he is going to think it is okay for him to say too. How is it not okay for him to say it to you, but for you to say it to him? He was using it to express himself since he can't quite get "I don't want to come to you and get my diaper changed. I am happy doing what I am doing right now."