R.M.
Sounds to me like you are handling it well....don't over react...stay as calm as possible...and just make him stick with it until he is done. I can't imagine that a 6 year old has much in the way of homework...so it should be an easy process for him to complete it.
Maybe you could entice him with a "reward" for finishing his homework...dessert....a favorite video...family playtime. Make it worth his while to get his homework done and out of the way.
If you establish a pattern now...he will have study habits that stand him in good stead for years!!! Could I suggest that you talk to his teacher after school starts...he/she should have some great ideas about how to make things work more smoothly without the constant battles.
R. Ann