I'm guessing they found out when he was young via check ups, there could be a mutation that caused his uterus to be connect through his an**. It is very possible but very, very, VEERRRYYYY rare, and many complications can occur. I saw a tiktok video of a man that got pregnant through an** s** (they showed proof). In the real world, they would need to have a cesarean section, but in this manhua, he could give birth via his an**.