This is a long post but please read it, I have several ideas that may help. I don't know the exact problem your son is having with the work, so I just tried to go over several possibilities. One, I wonder if your son is having trouble in an area that makes homework discouraging, aggravating, boring, etc. A child can be very intelligent yet still have trouble in some areas. In fact, some children can be considered gifted, while still having a learning disability in other areas. Even if it is much more minor than this, that could make all the difference for the child's reaction to his work. We all have strengths and weaknesses, and most people don't deal with those weaknesses very well. Let's say he's pretty smart, but has trouble with something like writing, putting his thoughts together on paper, holding his pencil comfortably, or any other number of things. Even something that seems very minor to us may be very frustrating to a young child. A child who is usually very smart may react much like your son. This does not always mean he just doesn't want to do the work, it could bother him to have such problems, to feel so frustrated, and thereby make him feel like he'd rather not try because it's just so frustrating. In that case it's probably best to find out what his weaknesses are and try to make some accommodations, just help to work through or limit those issues.
Another issue that is far different from the above could also be a factor for a smart child that feels the work is boring, stupid, or un-challenging. It's easy for us to say we should just make him deal with it, but many intelligent kids fall behind when they are not properly challenged, or the work is just boring. To be very honest, almost any child will have trouble pushing through work they see as boring, and at six it is probably a bad idea to force the issue. He is not at that age yet, maybe in a year or two that would be ok. Because he will eventually have to learn to do things he doesn't want to, no matter how boring or difficult they may be for him. Until then, it would probably be easier on everyone, and better for the child to make some minor adjustments to his work. Is it boring to him? Can you somehow make it more interesting without changing the bulk of the assignment too much? Is it un-challenging to him? Could you up the challenge a bit so it is more challenging, again without changing the bulk of the assignment? Just remember it's ok to challenge the child, but don't make it so challenging that it is extremely difficult, that will just discourage the child. Does the child quickly get tired of working on it? Could you allow a short break, a breather? Maybe a small snack, a bit of play outside, time to just sit and talk or rest. Small little breaks are very good at "recharging our batteries," whether you are young or old, it shouldn't really matter. Sometimes when we get stuck it is better to take a step back, walk away, take a break, stop thinking about it, and come back to the work later. It's also a good idea not to start the work really late, then the child will just be too tired to work productively and he will certainly let you know that.
Is the child having trouble with something in the assignment as I suggested in the first paragraph? In that case, I advise you to speak with the teacher about your concerns, take time to brainstorm ideas together, and come up with some things to help the child work through his difficulties, thereby making the work more manageable for him and making him more willing to put forth the needed effort. Both the teacher and parents discuss keeping an eye out for areas he may be struggling in, so that you can collaborate later and find more ways to help the child work through these issues. Also keep in touch concerning what seems to be working, this helps to keep everyone on the same page.
The best thing I can suggest is for you to be very observant. Try to figure out what issues are contributing to his reaction to the work. Then try to find ways to deal with those issues. Keep an open dialog with the teacher and anyone else that works with the child and have everyone try to concentrate on ways to deal with those issues. Some teachers are willing to make small adjustments to the assignments or have the parent make small adjustments if it doesn't dramatically interfere or change the bulk of the assignment. Some don't, just deal with that as best you can if it is the case, and try not to get too upset about it.
One more thing, if the child just wants something more interesting or fun then what is being assigned, try to think of simple ways you can make this work more entertaining or interesting. Contrary to the belief of some, it is OK for work to be fun and interesting. It can even be beneficial, because most children learn far more and far better when the work is memorable, interesting, and or fun. They also tend to be much less resistant to doing the work if it keeps them captivated. Also let the child give input too, he may have good ideas from his own perspective, he may pin point something that hasn't been considered, he may come up with things he finds more enjoyable, and he will probably feel much better about himself if his ideas and input are meaningfully included.
