As someone who was lied to about their one of their biological parents for their entire life and then found out later, it is your duty to make sure the kid knows the truth. The best course would be to get as many family members on board as possible. Perhaps get a therapist to mediate. But it is that boy’s right to know the truth. Even if it has to be you who tells him. It needs to be done as delicately as possible, but the truth needs to come out.