If this was a one off it may actually be best to let your son and his dad resolve matters between themselves and not make a mountain out of a molehill.
First of all it is perfectly normal for parents and children to have disagreements. There is a very fine line between "reasonable chastisement" (which is a defence in law) and the criminal offence of assault. All parents upset their children sometimes. Saying `no' and managing difficult behaviour is an essential part of parenting. Tired or stressed parents can lose control and can do or say something they regret, and may even hurt the child. If this happens often enough, it can seriously harm the child. That is why abuse is defined in law. The Children Act 1989 states that abuse should be considered to have happened when someone's actions have caused a child to suffer significant harm to their health or development.
Secondly, you weren't there and only have your son's version of events which could be exaggerated. You could be making a rod for your own back if your son thinks it is OK to rush off to the other parent when there are disagreements and he doesn't get his own way. Children need to learn about reconciliation otherwise they will have difficulties leading to dysfunctional relationships in adulthood. Sometimes children unwittingly contribute to disputes between separated parents. See;
http://www.mediate.com/articles/saposnekD4.cfm