It is always acceptable to use whatever tools you have available in order to protect yourself, but there ought to be an order to them/a set of circumstances that trigger them. The basic tools of self-defense are 1) words 2) appeal to a higher authority 3) physical force. If you are being attacked in such a way that you are likely to suffer permanent damage, then it is appropriate to skip directly to physical force to end the threat as quickly as possible. HOWEVER, in the case of your son, it is likely that the girl who is hitting him is not very able to inflict serious harm on him, so steps 1 and 2 should be tried first.
So. Next time this girl starts hitting him, he should say to her, "it is not ok for you to hit me, and I would like you to stop." If she does not stop, he can continue, "if you can't stop by yourself, then I will have to stop you." If she still continues, your son should next appeal to a higher authority, in this case his teacher. He can say to the teacher, "(name) has been hitting me, and I have asked her to stop, but she won't do it. Can you help me with this?" If the girl still does not stop hitting him after the teacher has taken whatever actions he/she chooses to take, then the next time the girl hits him, your son should show her that he is willing and able to defend himself, and that it has been his restraint as a respectful person, and not his weakness, that has allowed her to exercise this behavior towards him.
This advice, again, is for the situation in which she is not seriously hurting him. If she is - for example trying to push him off high places on the play structure, or hitting him in such a way that he is bruising or bleeding, he should not let her do this. In such a case he must hit her back. If she is big and strong enough that he can't make her stop through physical force, then he needs to fight back at least enough to be able to get away and get to a teacher. It is possible that your son will get in trouble with the school for hitting the girl back, even if they know the situation, but if that happens it is important that you stand by him and support his right to defend himself.
I would give identical advice if your son were being bullied by a boy. If the bully is not a serious threat, then words first. If he/she is, stop the threat first, and go from there.