All your nephew has to do is to log on under his RChat avatar and then use the reply button and post his comments just like I am doing now, any of the other RChatters who have contributed posts to the thread. RChat is a very public forum, and anyone can read what is on this thread, even if they have NOT signed on. So, you are the original poster, the person who started the thread. There's absolutely no reason why your nephew cannot post on your thread. He just needs to log on.
Cheers, JM