Oh S., don't you just love all the homework these days!
I am a work at home mom now (since my daughter was born), but I used to teach 3rd grade. The standards are so much higher now than ever before and depending on your district policies, their homework load my be too.
I really try to encourage positive reinforcement with anything first. Your son is probably spending all day looking at the same stuff he is bringing home to work on again. I'm sure he has "better things to do" than homework. I know I'm an out-of-the-box type, but try something like soccer or a game that he likes and use his homework in the sport. Like, for instance, I think they're working a lot on multiplication right now and timed tests? Try standing a distance from him and giving him a problem out loud, like 3x5, and he has to answer it by the time the ball gets to him. Then, he gets a turn and you answer. Just something to take the dull out and get more involved. Then when he goes it to do the "paper part" his confidence of knowing the answers is recharged and the activity got his blood flowing and I know it sounds crazy, but his oxygen increased and he'll be able to pay attention more.
This is just an idea that has worked for me in the past. Just getting them moving and out of pencil/paper mode is really important, in my opinion. Everyone is different and we all have to find what works for each of us.
Hope something I said helps :)
Best of luck and let me know if you need anything else.
A.
www.free2Bmom.com