is it normal for a guy to change his mind so quickly?

You asked him "what's up?"
He told you he's "watching tv."
Seems to me he was just answering your question, and you're reading into it wrong. There's no reason that conversation couldn't have been steered in another direction, i.e. "wanna hang out?" Or some such.
Whatever. Just give him a phone call and I doubt you'll have any more misunderstood communication.