Considering your child has a diagnosis of autism, I would first of all suggest looking into having some adaptations made to the work load. We are now homeschooling but when my older son (not asd but ADD/OCD/etc) was in year four, his teacher worked out a compromise for him when there was work to do and that worked extremely well for him. It was the only year he ever did well at completing homework.
I would think that the project is probably very overwhelming for him, even though you have broken it down. This is probably reinforced when you are trying to get him to do work on it all the time. (I'm looking at this from his point-of-view). Perhaps instead of a big project, make several small projects. I would definitely suggest discussing this with the teacher and perhaps see what could be worked out. Perhaps the teacher would allow your son to choose his own project in lieu so that he has more control. Or it could be some other compromise.
Your son should have an IEP (individual education plan) in place. If he doesn't then I would suggest looking into to getting one done. Either way, I would include this as a point that needs to be tailored to his needs and abilities. If it's a public school there is an autism unit of visiting teachers available to help with classroom issues too. I consider homework and projects as part of that. The visiting teacher might have other suggestions.
Otherwise, I can't suggest much else as we no longer have to deal with those issues.