I just started dating a couple with children that are roughly the same age. I think it's up to poly people to make good decisions about their sexual presentation and behavior. Every text you send, you have to think of it in terms of, "If Martha asks her 80 year old grandma or her 10 year old daughter to check her texts because she's expecting a message from Aunt Sally, would I still be okay with sending this?" Stops a lot of nonsense before it happens.
But your 13 knows your wife, you, or possibly both have you have railed or been railed by this young man. I'd say the worst thing you can do is beat it into the ground. Tell the simple truth. Don't blather on and tell too many details. Then get on with life.