I adopted (8M) when he was a baby from my sister. He's starting to put the pieces together and asking questions. How to handle this?

Zack was a teenage pregnancy. My sister and her boyfriend at the time were in high school and thought a baby would equal eternal love. That blew up in their face when they broke up mid pregnancy. I was 24 at the time and had a decent career. I offered to adopt him if I could and thankfully it all went well. It was this or he went into foster care.Zack has started to ask questions as to who his father is. My mother on more than one occasion has referred to him as Kayla's son, and then corrects herself to me infront of him. Kayla doesn't want me to tell him because she has a child of her own and she thinks it will upset him. To a degree I see that. Kayla just can't not act weird around him.How do you handle this? I knew it would come one day, but I never knew he would start asking questions this early. via /r/Parenting http://bit.ly/2DAOlT2