Hi Miranda,
I have a daughter, not a son, but I have been a fourth grade teacher for 8 years so I am familiar with the behaviors of 10 year old boys. One technique that worked wonders in the classroom could also work at home if you are willing to be persistent with it.
Have your son choose his favorite activity that he likes to do in his free time, i.e. video games. Each day, he starts with a set number of minutes that he is allowed to play his games, let's say 30 minutes. (You can set any time that you as his parents are comfortable with.) But the activity must be something that he enjoys/looks forward to.
Okay, let's say it is Monday. He starts the day with a clean slate, no matter what happened on days prior. So, he has 30 minutes of game time to look forward to later that day. Now, let's say he doesn't want to get ready for school in the morning - give him a set time he has to get ready - "You have ten minutes to get dressed, etc. and come down to breakfast." If he takes longer, let's say he took 13 minutes, then those 3 extra minutes that he wasted of your time, get taken from his game time, so now he is down to 27 minutes - no arguing/yelling, etc. He has made that decision to take longer...
Now, on the other hand, if he only took 8 minutes to get ready, then the extra two minutes he saved gets ADDED to his game time, so he now has 32 minutes of game time later.
You can use the minutes however you want, but just make sure the expectation is known ahead of time. Then he knows what is expected and any 'punishment' or loss of minutes is not a surprise, or reason for argument.
This works wonders in the classroom, and I have heard from other moms that it works at home too. It means constant supervision during "game time", so they don't take advantage, but it is worth it for the headaches it saves!! I haven't tried it at home yet - my daughter is only 21 months, but the rate she is going, I could see me using it soon!! :)
After reading some other mom comments, I wanted to add something. I do not think you need seek medical advice yet. I think he is being defiant because he knows you and your husband are going to repeat yourselves many times over. Once you cut back - ask him to do something, tell him to do something, then take the minutes, he will become a better listener. Kids are not born good listeners - they have to be taught. If you follow this routine, he will get used to it, and he will know you mean business. So many people just run to the doctor to fix things before exhausting all possible solutions. Don't give up, and make routines easy enough to stick to - be persistent and consistent!!
Good luck,
L.