I totally understand where you are coming from and I think I might be able to shed some light on the subject partially from experience and partially from my best friend who is a 4th grade teacher.
I will start with my experience, my son is in 4th grade and has been doing the science fairs for school since 1st grade even though it wasn't required at that age. When he hit third grade he had a teacher that openly picked on him. His science project that year was way over and above what the other kids in class did, yet she sent it home saying it needed more work. Believe me when I tell you I am not just being a mom and bragging about how great my kid was and how awful this teacher was, he did an electrical circuit that kids could test their steadiness skills and win a prize. He also built a board with color, photos and all the required hypothesis, data and conclusions required by the teacher. It was ridiculous. In short, I walked into this classroom with the project and made her show me exactly what was lacking on his project and had her show me specifically what the other kids in the class did that did not require a redo. She was unable to support her request to redo it and he got an A.
Now, this situation leads me to two comments, I do not think that all projects require a confrontation with a teacher. This teacher is a problem and has been for years. I do believe that if your son is asked to work further on something then you need to help him to clarify where the disconnect is. In short, teach him to take on the challenge and meet the requirements. Teach him to ask questions, not to just get disgruntle and angry about it.
One of the things you mentioned is that the teacher is adding requirements after the fact. I need to turn to a conversation I had with my teacher friend on this one. I went to dinner with her one night very frustrated that my son was getting what his school calls "correct and returns". Like you, I felt like my son and myself did not have the proper information to do it right the first time, like things were being changed mid-stream. My friend said that this was the biggest complaint she has from parents and she consistently finds it odd that the kids act like they are clueless on direction. Over and over again she will give the directions, tell them to write it down and over and over again they come back with their projects incomplete. In short, she feels she is being VERY clear on direction and half the kids miss it, so it is possible that this is a two-way street. The only way to find out is to talk to the teacher about your concerns.
One thing I know for sure is 3rd grade is the start of the transition from fun school to hard-core academics and 4th grade is almost brutal. You have an opportunity to teach you son a very valuable lesson. He will experience this type of situation many times in his life, even through his career life as an adult. Whether the teacher forgot to give full direction or whether he didn't remember them, the fact is this teacher has given him a chance to make his work better. I think if you have the chance to help your child learn to take what he has accomplished and push forward then you have a win win situation.
I read a book recently that covers the fact that a lot of times in our lives we settle for "good enough" and never reach our full potential. If only we stopped, looked at our accomplishments and thought "how do I make this better" will we ever really reach the excellence we know we can be.
One last thought, I know this is long, sorry, after 3rd grade and having such a horrible experience with my son's teacher I decided to pull him out of that school. This teacher had me convinced that he should repeat 3rd grade and he was too far behind (he did not demonstrate this to me at home or on his homework and had been in the GT program the year before). I had him checked by the doc and then had him tested by 4 different nationally accredited programs to see where his aptitude was. He scored across the board in the 85th percentile and higher. Not really a kid that needs to repeat 3rd grade. This year he is in a private school which was a full year ahead of the public schools on curriculum. I was horrified that this would be above his head and too much to ask of a 9 year old, but you know what? He prevailed. Finally in the 3rd quarter of this year he is on the Merit Role. Many times he has had to redo work sometimes and even work I don't think warrants a redo. Many times he is frustrated and upset that he did his best and had to redo it anyways but as the year moves on, he is getting less and less, and doing better and better work. Work I never thought he would be capable of at 9 years old.
Ya know what though, I am thrilled that I have moved forward and have told him to do it, I am so happy I am not selling him short. He may have done his best on the project but I know that he can rise to the occasion and do even better. Remember his best yesterday is not a good as he can do today. Every day these kids learn more and every day your son's teacher wants him to be the best he can be.
If you have to lose some time and help him on his project, then help him. Help him get it right, don't give him an excuse in the future for mediocrity. Teach him to do the best he can even when the person giving the direction is off a bit. Encourage him when he has to do work again, it may not be the most fun thing but his confidence and sense of accomplishment when it is over will be what sticks with him.
Good luck! Believe me I know how hard it is, sometimes I feel like I am doing 4th grade all over again. = )