As someone with a... unique name, this sounds like my reaction to hearing my name mispronounced in a way that has previously been used as an insult. It's likely you just accidentally struck a nerve.

It's less awkward if you don't continuously probe him and apologize again and again. Just make a very deliberate effort to never make that mistake again. Make an effort to perhaps invite him out to lunch at a place where you will not be invited to make an ass of yourself (i.e. don't invite him to a Japanese steakhouse owned by white people and try to pronounce the menu items).