He is an adult (albeit a young one) and it's his right/responsibility to be the primary liaison with his parents about this, not yours. I would keep a respectful distance from them and let him handle this. It is perfectly valid for you two to carry on in secret, because what businesses is it of theirs? From my perspective, though of course how he handles it is his decision, they have violated his trust so they lose out on the privilege of knowing about his life until they can deal.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.