This thread broadly breaks down into two camps - as these type of threads usually do -
those who think the action of the officer was unnecessary, inappropriate, and should be a discipline offence
and those who applaud his action as being correct under the circumstances.
I would merely add this - the action of the officer in headbutting the gentleman in question cannot reasonably be defended as being 'appropriate restraint' since I am sure it appears nowhere in any training he will have undertaken in dealing with this type of situation.
Headbutting is not 'restraint', it is assault, and had the gentleman on the receiving end been the instigator of said assault, I am sure all the defenders would quite rightly be howling for a prosecution.
The law msut be applied even-handedly, or it ceases to be the law.
The officer should be charged with assault, because that was the offence he committed.