So I spoke to the boy and discussed how one side is a fair bit bigger. He's not worried but wouldn't let me check. He did let dh have a feel though and it feels like the other side, just bigger. There's no pain or redness. We had a chat about the importance of self checking and letting myself or dh know if there is any pain or if lumps appear. Other than that I guess I'll keep a subtle eye and if the other one doesn't catch up within a few months then I'll broach the subject again.