My son was asking to borrow a baby sister

My son who is 6 dreams of having another sibling. I catch him and his brother praying in his room for a baby in mommy’s belly. We have been ttc for 13 months. Yesterday we were w family, and he asked his cousin if he could borrow his baby sister, bc he really wanted one. My heart shattered, needless to say I have been crying on and off all day today. I try my very hardest to stay positive and strong, but at 13 months ttc it becomes very challenging some days.