Find out why
The first thing to do is to find out why he's running away and crying. I am deeply concerned that no-one in your family has apparently even thought to ask him that. If you had, then I would have expected to see that as a major part of your question.
If in fact you have, then please edit your question to give us this crucial information.
And if you haven't, find a time when there isn't homework happening and your son is calm, and talk to him then. Make sure you're clear that it's not about telling him off, it's about finding out what he's feeling and how you can both solve that together. Or get someone else to talk It through with him, if he won't talk to you.
In my experience as a parent though, there are three really common causes of this, and natural solutions to all three.
Fear of large amounts of work: Teach him divide-and-conquer
Children don't have much of an attention span, and a large chunk of work can simply frighten them. Teachers break down class work into small, manageable chunks which the children can deal with, and that's the same approach you should take. Look at the assignment and see how you could break it down. If it's 20 maths questions, tell them they can have a break after 5 questions, get a cookie or a drink or something, then get back to it. If it's a writing exercise, let him brainstorm some bullet points and then you can divide it by those. At 6 he shouldn't be getting much homework so you can also split it over more than one night.
This is how you do it at work too, of course. You work while you've got focus, then you take a coffee break, then you get back to it. His period of focus is just shorter than yours, because he's 6.
Frustration with not being able to do it: Mentor him
Children can easily get frustrated if they can't solve a problem, or if they repeatedly get it wrong. Teachers aren't always good with giving kids the individual attention they need to grasp things.
At 6, your son absolutely should not be doing homework on his own. Someone should be sitting with him to check what he's doing, and mentor him with his work.
You need to be very careful not to do the work for him. As with all mentoring, you need to watch how he's doing it, and when he goes wrong you need to have strategies to deal with it. If he's getting mental arithmetic wrong, for instance, get him to go back to number lines.
I remember having a conceptual issue with subtraction where I was counting the initial value and everything was off by one, because I literally had not connected subtraction with numbers to "taking away". Had anyone thought to give me a number of stones and say "take away 7", I would have got that concept more easily.
Again, we're back to solving this in the same way you would at work. You don't just throw a teenage apprentice at a metal press and say "make me this", because they'd lose fingers. You show them how to do it well, and then you let them do it under supervision until you can see that they've got the idea.
Fear of failure: Get him used to checking his answers, and show him how to check them
He shouldn't ever be afraid to get things wrong. Make sure how you pick up mistakes is constructive. But more than that, get him to do his own checking.
If he's doing subtraction exercises, have him get to the end and then add the subtracted value to the result. If they come out the same as the initial value, great. If not, try again. If he's got a writing assignment, get him to read it through afterwards and see if he thinks he's got his spellings and grammar correct.
Again, I take this back to work. I'm an engineer, and I've worked on stuff where if things go wrong, people die. So I check what I've done, and then I get someone else to check it too. The principle is that everyone makes mistakes and it's totally normal, so we do some checks to make sure we're getting things right.
Keep it constructive
This is the overarching principle. If you want to relate this to work, professionalism doesn't just mean doing a good job yourself, it means working constructively with other people too. You don't insult them or shout at them for making mistakes, you point out the issue and work together to resolve it. If they get emotional, you don't raise your voice too, you keep calm. All this translates just as well to helping a child do their homework.