Maybe the homework is the wrong homework? Even if it seems easy to you, maybe to him it is difficult, or he senses he is going to fail, or that you might ask him to do more even if he does a little bit. The teacher might need to set him different homework for now, if he has a block about doing any at all. Maybe it could be cutting words out and sticking them in, or colouring, or talking about something with you, rather than something that requires what he regards as work or writing. Maybe you could scribe for him if it is a research project.
Sometimes we have had success (and this is with a 12 year old) in setting the timer and saying, in 10 mins we are stopping. And break it up into very small chunks, total time on homework is 30mins in 3 10 mins sessions. Then you break the extreme fear and resistance cycle.
No criticism at all whatever he does, even if it is completely wrong, untidy, rushed, mispelt. Nothing matters except that he actually sits down and does something anything. Ds now does his homework much more diligently because we have been so careful not to criticise the presentation and allow him to make mistakes. Also to let him do the minimum - often he is the one now who wants to go that little bit further to please his teacher.
Teacher needs to discuss with you what the point of the homework is at this age. She sets homework for everyone, so you need to tell her that differentiating for your child is important, which might mean NO HOMEWORK, and some other task which is on topic.
Chewing is apparently good whilst writing, and a small trampoline to bounce on between concentrating. Forcing them to sit down never works. Snack and drink at same time often helps, straws, sucking etc.