Personally, I think Rex is more shallow than what people think he is.
If you look at all the times he's "stood up" to people, others have put him up to it...Last night and previously was as a result of what Becky & Luke said to him.
Previous, it's been from what Mario or Luke has said to him.
People think he speaks his mind, when really he is acting for a mouthpiece for other housemates...Whenever he's confronted he always goes on the back foot - similar to Luke.
I've changed my mind, Want Rex to win now, comedy gold!
He was just agreeing with what Bex and Luke said anyways, he already had his own suspicions and was happy to tell Rachel what he thought about her (Remember when he made her cry the other night!) and seeing her VT just confirmed his suspicions.