I think I treat my son well
How did you come to conclude that ?
I think I'm a good father.
It's good to reflect on what kind of parent you are. This way you can be aware of any good or bad things in your parenting style. If there are issues you can fix them in order to be a better parent.
I've decided to show these assholes who think I'm not a good parent.
It would be better to ignore people who think you're not a good parent as long as your son is healthy and happy. As long as you know you're giving your best to your son as a father you don't need to worry. There's no reason to start a conflict.