I'm in America instead of England but here it would depend on if someone would call the cops & if your dad would press charges on him. The police would let him go if their investigation determines that he was acting in self-defense because of the way your dad was acting with him like threatening him, pushing him. Otherwise he would probably be arrested for assault & let out on bail till going to court unless police or judge felt he could be suffering from major psychiatric problems & then they might send him to a mental facility for an evaluation & treatment. After trial he might could serve jail time, fine &/or community service. Your dad could also sue for his medical care, lost wages, pain 7 suffering, & if doesn't go away he could sue for becoming disabled due to the incident.