I teach in a state comprehensive. I don't set research because too many kids just copy and paste without understanding. And I don't want to see their parents' work.
I go through the homework in the lesson. The kids do it. When it comes in, I check it's done properly and then I go through it in class. If they've misunderstood, they correct it.
I don't have time to mark homework, so I set stuff that I can do in class. If they get home and find they can't do it, it's their responsibility to come and see me for help.
So, I would send your son back to his teacher - s/he needs to know that he needs help. Even in a grammar school, there will be some range of ability - teachers will expect some Y7s to need help at the beginning.
If it's not just one piece of h/w, but most of it, contact the school. I have to lower my expectations for some lower attaining students. It's not that they're 'thick', but they take longer to understand something. Homework should be useful reinforcement, not torturous.
Don't blame the teachers before you speak to them,. Most will want to help.