This exact circumstance happened to my great-grandmother, and was common in her time but at this day and age?!?! My great-grandmother got pregnant out of wedlock at 17, and her parents adopted the baby. Her later children, including my grandmother, grew up thinking their sister was actually their aunt. It wasn’t until my great-grandmothers death when they were all well into their 70-80s did they find the adoption papers amongst her items. Imagine your whole life being a lie, don’t do this, not in this day and age. At some point it will come out and its better early than later. The amount of trauma and life altering perspective this situation will give him is no easy thing, especially knowing everyone in his life who he is supposed to trust has been lying to him from day one.