Is it permissible for a father to marry a girl who was breastfed five or more times with his son?

There is no legal impediment to a man marrying his son's sister or his mother through fosterage. You may marry the girl who was breastfed with your son; she is his foster-sister, and she remains a non-mahram to you unless she was nursed by a woman who would make her a mahram to you, such as your wife.