I suspect three factors may be at play here;
1. He's at that tween age where he's testing limits and freedoms. Get ready...as he gets older and school crushes, and cars come along it'll get more challenging. Hold to your guns!
2. He's indignant that you're expecting him to clean on top of helping with the younger siblings. Right or wrong, he might think you're taking advantage of him because you're suddenly upping expectations. I gather from what you have written, because he helped with the little ones in the past, you let other chores slide, but now you want and expect more from him because both he and the younger ones are a bit older. The switch up might be confusing or understandably frustrating him.
3. If your other kids or even you and your husband don't have assigned duties and expectations, that means there's no consistency in overal expectations within the household. If there's no consistency in expectations for everyone at home, he's merely following example.
In short, I think the solution to your problem is going to be setting up a chores schedule/calendar for the entire household (that means everyone from the toddler to the parents)and reinforce it with a rewards system of your choosing.
I think children don't do so well when they are "told" to do something. They need to see and be shown what to do. Using a set schedule will help you to effectively establish and enforce the all too important and necessary consistency within the household for expectations, it also visually shows everyone what your expectations are, and will also equal the playing field by showing everyone has a job to make the house comfortable and nice.
Be sure to tell the kids this is how to make the home a happy and clean place to be. Make duties age appropriate and explain this to all involved so no one argues the plan is unfair. Duties for the youngest can be as simple as picking up clothes, putting toys in the toy box, or giving the dog water. Middle to older kids can do way more, including help with dishes, floors, yard work, even laundry.
Be sure you as parents are includ yourselves in the duty mix. Keep in mind, if there's a way you want things done, you are the one who must set the example. Don't nag, don't chastise, just reward well when it's done the way you like. Competition among the ranks will naturally occur and in no time your expectations will be met! ;)
Remind the kids that "everyone" will be expected to chip in, making it fair. Make the duties fun, and avoid calling them chores. Make sure all are made accountable by checking off on the weekly list their accomplished duties.
If someone does not keep up with the schedule, there should be consequences or some sort of penalty such as a removal of a privilege or docking in pay from the "reward system."
Good luck and hope this helps.