My 1st grader has been writing about how he wants a mom. How concerned should I be?

My son is 6 and in first grade. They do free writing during the day and one of the main topics my son focuses on is wanting a mom.The teacher emailed me after they made stories. The goal was to write a page every day and have them connected to each other. The prompt was "If you could have one wish, what would that wish be?" My son wrote about how he would wish for a mom.It's just me and my son and has been since he was a year old. His mother left and my son knows that. We have talked about why she isn't around anymore but I haven't gone into specific details because I don't know them. We weren't together when she left. She pretty much dropped my son off at his grandmother's house and never came back. I know that she is still alive and tried to get in contact with her when my son was around 3, but she told me to never speak to her again and I haven't since. It's hard to answer my son's questions when I don't know the answers myself.His teacher wants him to meet with the school counselor twice a week. I am okay with this but should I be looking deeper? Into therapy outside of school? He is a great kid but probably has some attachment and abandonment issues. He still sleeps in my bed at night and doesn't like for me to leave him overnight, so we haven't spend a night apart since his mom left. I have just accommodated those because I didn't think they were a big deal but with how much he has been writing about his mom, I feel like shit. Like I missed all the signs that he isn't okay. via /r/Parenting https://ift.tt/2I4c80t