Definitely NOT too young to tell him off!
Look, we all have our different ways of disciplining our kids.
I am pretty strict and would absolutely not tolerate this.
We have a naughty corner in our house. They get told once firmly, "no, we do not hit/pinch/bite, that is NOT good behaviour, if you do it again, you'll go in the naughty corner" and if it happens again, that's where they go.
A minute for each year of their life.
Then you go to them, get down to their level and say "I put you in the naughty corner because, blah blah, now what do you say?"
They apologise (if they refuse, they stay there until they're ready to) and then you cuddle!
This works for us. My 3 year old has been the most trying of my children, but consistency with discipline has really helped.
Some people will probably say he's too young, explain gently.
I say the earlier you start, the easier it is. Otherwise youve got a headstrong 3 year old terrorising kids at playgroup.
And trust me, that won't be good for your child either.
No one will want to be friends with a child who is being mean.