good god sounds horrendous.
OK, I think there are multiple issues here tbh.
Firstly, I think it is important to try and establish whether this HT resigned or was sacked, because if he was sacked then IMO the question needs to be raised as to why he was appointed at another school, and whether there was some pressure put on the governors by the LEA to appoint this particular head. Even if he resigned there's a chance that the LEA might have applied some pressure in order for this head to go quietly from one school to another. The LEA has no say in who a governing body appoints, but if the governing body is not a strong one, then it is quite possible for the LEA to put their agenda across and for the governors to comply, perhaps because of lack of experience, or even lack of candidates, and therefore an unwillingness to fail to recruit.
Do you know what school he came from? If so it is worth looking at their website and their ofsted reports to see what action might have been taken at his previous school.
What form have the complaints taken? Have parents spoken to him directly? if so, and they are still not happy, the next course of action would be to speak to the chair of governors.
Before a head can be sacked there has to be a disciplinary process. I am unsure as to how this can be brought about as I haven't had to implement such a process. But I can certainly try to find out.
I would imagine though that it would be an ongoing process which would involve self evaluation and performance management meaning that certain targets would have to be met over a period of time before a sacking could take place. Unless of course the incidents were so serious as to warrant immediate dismissal.
Hth.
PS: I am a recently appointed chair of governors who has ironically just had a part in appointing a head teacher.