Son of my cousin

My dad has a brother, who in turn has a son. What's the relationship between that son and myself? The answer is fairly easy - we're cousins.

My sister has (or rather, will have) a son. What does that make me? Another easy one... I'm his uncle and he will be my nephew.

So far, nothing new, but now let's mix it up a little... My cousin has a son. What does that make me? The answer, according to wikipedia, is... we are first cousins, once removed.

Hmm... and what does that make him and my sister's son?