"I say first: What do YOU imagine homeschool to be? What do you want your days to be like? What kind of things does your son enjoy learning about??
From there....that's how you figure out where you need to go."
If you're going to take on the homeschooling project, then you need to have a vision. Get yourself some books about homeschooling and work on your vision. Let your son finish the school year unless he's actually requesting to be taken out of school, because why introduce a paperwork hassle into your life for no good reason - but if he IS requesting to drop out of school, then it's work the paperwork hassle to show him that you're taking this seriously and you are in his corner, and you can contact an accountability group in your state for information on how to proceed.
Before you can know what kind of curriculum (if any) to purchase, you need to know what kind of homeschool experience you want to provide. So read, read, read - books, blogs, message boards, etc. At some point another parent will describe their child's traits and what worked well for them and you'll say "Ah-Hah! That's my kid!" and then you'll have a curriculum suggestion.
I am a classical homeschooler, so I hang out on the Well-Trained Mind
forums. I'm also a fan of the Latin-Centered Curriculum
. You might browse those forums and see if the parents there seem to going in a direction that appeals to you.