Has he eaten any of those Puff type (can't think of name) snacks? I've heard people say their kid's drool would turn blue after those. Have no idea what is in them.
The only other thing I can thing that sounds familiar is I've heard of pillows turning blue/purpleish after drool. From iodine in the spit?
I would ask your pediatrician for sure next time you are in.
I use Stain Away in gel form here (green bottle). And I use the clear clothing detergents. I was getting blue stains on my clothes from my old detergent - would just appear after the wash, even though I wasn't mixing clothes. So maybe try that just in case it is your detergent.
Good luck :) hope you get it figured out
ETA: Two other things I thought of - his toothpaste (maybe change that to see if it improves) and if he takes a multivitamin (or liquid one?).