My friends son is 3 1/2 (nearly 4) and hits people when he is angry or upset. He did it the other day and hit his gramps. My friend makes him say sorry then says it's because he's tired. I was talking to my DH and we thought he should have it explained every time why it's unacceptable or at least punished (ie time out). We have a DD but she's only 16 months so am sure we have all this to come 
He's not got any SN.
WWYD?