Yep, my 4yo said similar things when he was 3yo... I found that it was often mimicking what others (his older sister and DH) had said to him.... things like "you're a poo-head" (charming, I know, courtesy of 12yo DD) and "Nick off" (DH at his worst) *sigh*. It's so saddening to hear my sweet little boy speaking that way and I have very little control.

I think half the reason they say these things is to test your reaction... when it was said to them it obviously hurt them... they don't know how to deal with that hurt so they are looking for ideas (all subconsciously) so they repeat them to you (who in his eyes holds all the solutions to the world's problems! ). So, I think it may have been said to him at some stage... and the way you deal with it could be the way you would like him to deal with it. Maybe just stopping what you are doing, making eye contact and clearly saying that you don't like it and if he persists just escorting him to his room (all done in a neutral, calm demeanour). I guess you could just walk off and ignore him, but that would be essentially doing what he demanded.

Anyhow, just my thoughts, always the amateur psychologist! LOL :P