I'm 42 with a 2 year old and a 5 year old. Keep in mind that he's saying "no" and getting it from somewhere. So he must be hearing it a lot. With both our girls, we've tried to attach an action to what we're telling them not to do. If they tried to go touch the stove, we say, "No touch - hot" or if there's something "up high" they want, instead of saying, "oh no...." we'll tell them, "Oh, that's a Mommy-do." We purposefully did that to keep that "no" from coming back to us, and it's worked pretty well, but we do get it back sometimes.
When you tell him it's time for bath, and he says no, we usually tease the kids and say, "I'm not asking you, I'm telling you....it's time for a bath." Actually, we've never had problems getting them to want to take a bath - bath toys are wonderful things....even if it's empty plastic cups or the little boats you get at the dollar store, they love taking baths. If you don't make it fun, they aren't going to want to do it. Now, if we tell them it's time for a bath, they yell at the top of their lungs, "Time to get WET AND NAKED!!!!" and run upstairs.
We've also tried the route of..."I know you don't want to take a bath, but we need to. Otherwise you'll get real icky and oh, you might even start to smell pppppp-uuuuuu..." and they'll giggle.
For what it's worth, and good luck!