I think you're putting the cart before the horse. At 8 years old, he has a lot to learn before getting to the TMI part of your post. Have you discussed his underwear with the doctor? Have you studied physical development at all?
Just to give you an idea, sex ed wasn't taught in our school until 6th grade. Sure, you can teach your child at home, but you don't need to talking about what masturbation is to an 8 year old. If they are messing with their genitals, you tell them "We don't do that in public - go to the bathroom." You don't tell them more than they are developmentally ready to understand. They ARE ready to understand that they don't touch their genitals in public. And it's boring in the bathroom, so they won't stay in there long and will find something else to do that isn't associated with their genitals.
There are plenty of parents who don't tell their children everything they need to know. They get them books. The librarian can give you ideas of good books FOR THEIR AGE. The books offer good starting points for discussion.
All you have to do is read the books before giving them to your child. Then you have the vocabulary. What's so hard about that? You may have to learn the sign language for that, but that's part of you learning to talk to your son and your job - learning to sign.
If he's becoming withdrawn, it's not about your lack of sex ed. Stop worrying about the sex ed and learn to communicate overall with your son. Find a support group and talk to your ped about who you can work with.