Do you remember when your son said his first word? How do you think he was able to do that? Young ones are constantly absorbing everything from the world around them. They will pick up everything you say and annoying, how you speak. I younger brothers (twins) used to do a similar thing. One of them has a stutter but not the other one. The one without the stutter would fake a stutter now and then and I think it’s just a thing to fit in. It’s not done maliciously, it’s just them thinking it’s a normal thing