You are absolutely right. Do not dignify PewdiePie's video with a response. Although it would be understandable to want to sue PP, legal action has a tendency to ruin people's lives...sometimes even when they win. Just keep doing what you are doing, make great videos, entertain millions of people, and have a great time with your son.
By the way, I took a look at your channel because of this thread. I was almost scared that I would see a spoiled brat who clearly was being manipulated to promote toys. Quite the opposite, Ryan comes across (at least to me) as a really lovely, genuine, happy, well adjusted kid who is having a great time with his youtube channel. Sincerest Compliments.