Keep it simple - "When you were born, we thought it was the best choice, just like your daddy is circumcised (I'm making an assumption), when your brother was born, we learned that it really wasn't necessary so we decided to just leave him as he was born. There are many boys who are circumcised, like you, and many boys who are not circumcised like your brother - everyone is so different in so many ways, and that is just fine. Isn't it interesting how different kids have different hair, eyes and even skin color? Boys have different kinds of penises too."
If there is a follow up question like "why did you think it was the best choice?" then just offer a few things - "well, we thought it might be easier to clean, but then we learned that it really doesn't make a difference"