Or, the dad IS a bully.
So, what do you do if your kid is getting beaten, simply because he's smaller than the bully? If you talk to the teacher, the bully will typically take out whatever repremand out against your kid - a beating with a warning not to tell on him again. The cops getting involved has done nothing but to encourage the bully (See what happens if you dare touch me!, I can punch you, but if you hit back ....)
So, what do you do? This was a direct approach, given in a manner that the bully will understand. Bully's understand fear - that's the language they opt to communicate with - usually they are the one's who instill fear in their victims.
If the Dad WAS a bully, the bully would not have been warned or threatened - he would have been beaten. He would have black eyes, maybe a broken arm and cracked ribs - and then given a warning that if he said anything - the next 'visit' would result in his death. This would be followed up with a punch to the mouth, to reinforce the threat. That's the way bullies communicate.
As the case stands now - Dad tried a warning that was 'civil'. The next step will likely result in a life altering maiming or death of the bully. Or, Dad must accept his son's beatings and say nothing. Dear old Dad didn't initiate this, or provoke this - he's doing what he can to stop this from escalating. But, left alone - it will escalate in a way the Bully will not recover from.... it's inevitable.