Should I explain this to my step son and if so how?

My step sons goes to a middle school for special needs students. He on the spectrum. That school stops at 8th grade so he's going to regular high school next year and I'm really worried about him. They went on an outing today to the park and when he got back he said "Aunty mama(that's what he calls me for some reason lol) what does retarded mean?" I said "Why do you ask? " and he said at the park he bumped into a girl and she got mad at him and her dad told her to let it go, and that he's with the retarded kids. I tried to change the subject but he's been bugging me about what the word means. Should I tell him or let his dad tell him. His dad won't be back until tomorrow. And even if I do tell him what am I supposed to say?