Anonymous wrote:That is so cruel. I don't care if he is only 3. You don't do that.

It sounds like OP's kid isn't making fun of his friend. As an adult, I find that I tend to adopt the speech patterns of the people I hang out with. It's how I lost my NY accent after a couple years in VA and why I say "y'all" when others are doing so. I'm not making fun of Virginians, I am blending in. When my kids were little, they would speak the way their foreign nanny (who we all loved) spoke to them. It is the same with this 3 year old. He is speaking the way his friend does. It isn't cruel, it's assimilation and it's human.
That said, I agree with others that OP should work with her son to pronounce the words correctly.