Having been through a not dissimilar experience, I would not tell him. We got the early call about a possible birth sib long before we were ready to think about a second. We started getting our paperwork together and mentally rearranging our expectations, but that was the end of it. But we decided we were now ready, and when our son was 18 months we accepted the referral of another beautiful baby boy. We came to regret telling our oldest, because that adoption almost didn't go through. He would have been devastated - we didn't realise this, we thought he was too young to understand and be hurt by it. But the second photo we got showed two infants; he excitedly screamed "baby!" the instant he saw it but he didn't appear to even notice the other infant, only his baby. It was all he talked about, just beginning to talk. "Baby home?" "Baby ready?" Everywhere we went he brought his brother doll, who sat beside him in his high chair and car seat and stroller. He pointed to every plane in the sky and asked, "Baby?" We underestimated him. He knew his baby and would not have been able to process it had we started again with a new referral. In his mind, from the moment he saw that first photo he had a brother and he loved that brother.