My little boy wants me to adopt another child how do I tell him this?

My little boy wants me to adopt another child how do I tell him this: My little boy just turned 8 and he is the best thing that has ever happened to me. He is my miracle in life. Anyway, 5 years ago I had a hysterectomy. He has been asking me for the last 3 years if he can have a brother or sister, if I would have another kid. I told him I cannot have anymore kids (He wants me and my boyfriend 2 adopt a child. His real dad took off & I have a protection order against him anyway, well did until last month it was up). Anyway, so he asks me if I will adopt. I try to explain to him that it is expensive to adopt and ithere is a long waiting list and I just cannot afford to adopt, etc.. He is still a child and does not understand not being able to afford. I try to explain to him that he would not have all the stuff he has if he had a sibling, he would have to share. Is there another way of kindly explaining it to him? I feel bad that I can't.