That's a real douchebag move by him but many resort to such childish mimicking when feeling threatened or afraid, like when losing an argument. Personally I wouldn't take it so hard because it's not a direct insult to you as a person, just a poke at something external that you don't like about yourself. Pain comes from your insecurity. However if you want to quickly get back in such situation, to show that you don't care and feel more secure about it, you could always fake stutter back a jjjjerk or fffuck you while staring at the person. Worked for me in situations where I didn't get angry enough (never stutter in my angry voice). Hope this helps!