We have struggled with this with both my children, but especially with my son, now 10, who has ADD. I have combed the web, read books, listened to podcasts, and talked to "experts", but no one really seems to have an answer for this problem. Most of the advice I find about doing homework is about organizing it and prioritizing it, not actually getting your child to DO it. My son also hates writing. We have at times sat for an hour while he has managed to write three sentences. Clearly something is amiss, and I have many times wondered if we shouldn't just throw in the towel. If it is that painful for him, then maybe we should wait a year or two until his brain matures and see if it gets better.
Really the only thing that has helped is medication. When he is on Concerta it is still not easy for him to do any HW, but it is a teeny bit easier. Is your son on ADHD meds?
My son himself says that he wishes he could do his HW at school, where he has some support, and I really think that for children like these the school should arrange to give them supervised help from special ed teachers. To me it sounds like if your son is actually crying and fighting that hard (been there!) it isn't worth it. It cannot possibly be an overall positive in your son's life to do the homework. Can you talk with the school? My son's school has been somewhat understanding, if not completely.
Once I tried to convince my son that while doing his HW he could "look forward" to something nice afterwards. "I don't like looking forward to things," he said, "I like having them NOW!" ADHD in a nutshell!