You may be trying to scrub off staining that is from something else, like antibiotic exposure during pregnancy, or even earlier in his life, so the colour may not be anything you can do anything about.
In your place, I'd get a cheap moldable sports mouthguard for him to sleep with... they're between $2 & $5, so they can be lost and replaced often. They're a whole lot cheaper than professionally-molded, and your lad's jaw will be changing shape regularly, so he'd need a new one every few months anyhow.
Has he seen a chiropractor? He may have muscle tension in his neck or shoulders that is provoking the grinding. Alternatively, you could look at the book Baby Massage and help him fall asleep with a last-thing neck and head massage that may alleviate the stress, if that's what's causing the grinding.