whether or not the guy was messing around at first doesn't matter. The dad said to leave him alone. It could have ended at that but obviously the guy was up to no good if he turned it into a physical fight. You don't mess with people's kids...
The dad was rude, he could of politely told him to stop but didn't.
Fairly sure politeness goes out the damn window if you see a creepy drunk guy physically touching your young son without permission. Please stop trying to justify it.
