If he keeps scratching at one he might have an infection.
We started no right from the start. Obviously at first it's not effective but over time they learn and they learn pretty quickly actually. As they get older they are going to do stuff. Like throw their shit if you let them. It is up to you as a parent to stop them, tell them no and continue to do this until they get the picture that isn't what they are allowed to do. If you allow them to do whatever, they will do it. That might be screaming and yelling in public, throwing fits etc. People seem to think it is "normal" for kids to do this stuff yet everywhere you go that stuff stands out as pretty abnormal for the most part. You have to teach your kid everything, there is no such thing as common sense. If you start early correcting behaviors that are unacceptable you won't have to deal with them much when they get older. Consistency is key. Based on what I've seen, admittedly anecdotal, it is much harder to reign them in after years of ignoring behavior than nipping it in the bud. YMMV.
As an example, one set of friends of ours are shitty parents. They let their 5 year old run WILD. Scream yell, push, wild wild wild. He is a good kid, good heart, but he is just nuts. This is so bad that they are the black sheep of the group and when people find out they are coming they sometimes decide not to come over it. How do the parents react to their kid? They ignore him. He can come over here and he will be hell on wheels for 2 hours straight. Not normal kids stuff, at least not normal in any house I've seen or grew up in. At the end of the night he accidently knocked my daughter face first into the end table in front of everyone. What was the reaction? Well dad has obviously had enough and fucking flipped his shit, screaming (made me cringe) how he shouldn't be running in the house and yelling like a banshee, he slaps him real good and makes him sit on the couch. His yelling was colorful, expletive laden and full of anger as he obviously had been stewing on this all night.
Well, talk about mixed messages. You act like a heathen for 2 hours and no consequence. Then doing what you've been doing you end up getting whacked and yelled at. That is bad parenting and bad spanking but that is also very common.
I have no issue disciplining my kid, up to and including a smack if necessary. However smacking in anger or flipping your shit over something you allow them to do and only flip out because your dumb ass didn't see the consequence soon enough is just shitting reactionary parenting.
