How old does a baby Mallard duck have to be before it can go in a pond, or kiddie pool?

I have a baby Mallard and he always climbs into a small water bowel that I have for him. Would it be alright if I put him in a kiddie pool? He's very small. I found him on the street without his mommy, and he was going towards a very big street. I've been feeding him bread and I've been wondering if he'd like getting into some water where he can maybe swim.