First, do not be scared. You are a beginner and you are learning, too.

Be easy on yourself, AND your son. Homeschooling is new to all of you.
I do not know why your son is behaving the way he is because I do not know what he has been doing behavior wise. I have learned one thing for sure, and this may or may not apply to you and his father, but in my years of homeschooling, I have found that sometimes I am the problem. My behavior has to be every bit in check as my daughters. I can't expect a certain behavior from my daughter if I am not behaving myself. In the early days of homeschooling, I would easily get frustrated. Our frustration rubs on our children every bit as much as their attitude rubs on us. If you already feel that you made a huge mistake in your choice, then your son probably does, too.
Here are a few questions for thought:
-Is your son normally a winey and noncooperative person?
- Did your son want to be homeschooled?
- Are you teaching according to how he learns, or according to how you learn? This is a big one for homeschoolers. We forget that our children do not always learn the same way we learn or teach. Some kids are eclectic, traditional, hands-on, audible leaners, etc...
- Is the work confusing him, too difficult, in turn frustrating him?
- Was the assessment difficult for him? It doesn't matter how easy you think it was. Was it easy for him, or is he simply being lazy and complicated?
- Have you sat him down, along with his dad, and had a loving heart-to-heart as to why he is not being cooperative? This is assuming that the work is easy and that you guys didn't trigger any frustration in him. He could give you far better insight than we can. Not that your questions aren't welcome.
- Are you showing frustration?
- Does he simply not want to complete the work?
I have been homeschooling for 11 years and I am still learning.

The one thing about homeschooling is the dynamics are SOOOOO different than the dynamics of a public school setting. Study your son. Observe what he enjoys doing. See if he enjoys reading silently, out loud, with you and his dad? Get a feel of his strengths and weaknesses and build around that.