My uncle plans on punching his 18-year-old son and I'm going to stop them.
Why is your uncle going to punch his son?
Because he has done it several times before on different occasions
His father really shouldn't be hitting him. You're right in wanting to protect your cousin. I'm sure he'll appreciate you looking out for him. If this continues, you should probably contact the authorities about this. It's not okay.
I'm doing my best to convince him not to punish him. I really don't want to see him get hit.
Definitely approach your uncle in a calm manner to try to de-escalate the situation. Make them see reason and most likely this would result in a positive outcome.