A.H.
Instead of asking "do you want a bath?" say "it's bath time, go get in the tub. Do you want the red towel or the blue towel?" If you don't ask ??? about the chore there is less room for the word no, but you still give him a choice about the fun so he feels in control. Kids act out and say no when they are trying to find a sense of control in their lives.
If he is still saying it no matter what choices you give, What we did with our son was "Who's Mom and Dad (or the parent)?" he would say "you" and then we would say "who's the child?" and he would say "ME!" and then we would ask him "who makes the rules?" and he said "you" and our response was "So go get in the tub. (or do what ever)" and we used this all the way thru preschool. It took some training to get the conversation down but it really established our roles, and ended arguments.