My daughter used to self harm and her arms are left terribly scarred. Legs too, but obviously much easier to cover up. She works with children and I was concerned they might be frightened or ask questions, as she doesn’t try to conceal them at all.
So far only one girl has commented, my daughter said she gently stroked her scars and and asked if she’d hurt herself (not asking if she self harmed, more if it was an accident, I guess), and daughter told her that she did but she was all better now.
I think your son will just see them as part of his mum. Someone that he sees all the time, and loves so much that they’re as familiar as your nose. He might ask when he’s bigger, and you can tell him properly, but for now, if he asks, just say that you hurt yourself a long time ago, but you’re all better now.
I really hope you are 
I’ve got some visible scars, chicken pox on my nose, a horrible one in my eye from surgery, and a 5cm long narrow burn scar at the top of my chest. My children have never commented. I’ve asked them about the one in my eye as I’m self conscious of it, but they say you can’t see it. Yours will fade too.